feat(network): pluggable connection filters; file blocklist + contribute-first CrowdSec (#2542)

Reshapes IP banning around one idea: **core owns the question, content owns every answer.**

Core gains a single accept-path seam — `IConnectionFilter` — and loses everything that used to implement one. The firewall moves to UOContent, a new file-backed blocklist joins it there, and CrowdSec is repositioned from an in-app enforcer to a contribute-first reporter.

## The seam

```csharp
public interface IConnectionFilter
{
    string Name { get; }
    void Configure();
    void Start(CancellationToken token);
    void Stop();
    bool ShouldDeny(IPAddress address);
}
```

The accept path went from hardcoded branches to one question:

```csharp
else if (ConnectionFilters.ShouldDeny(remoteIP, out var deniedBy))
{
    logger.Debug("{Address} denied by connection filter '{Filter}'", remoteIP, deniedBy);
}
```

Filters register during the Configure sweep. The registry is a plain array walked by an indexed loop — no enumerator, no closure, no allocation — and the first denial short-circuits. An interface dispatch is noise next to the `accept()` syscall, so pluggability costs nothing measurable on the path that has to survive a DDoS.

Whatever a hit implies — persisting, promoting to an OS bouncer, contributing to the ban channel — is the filter's business, not the accept path's.

A filter that throws is **unregistered and the connection fails open**. A filter that faults once faults for every subsequent connection, so leaving it registered means an exception and a log line per accept — exactly the amplification an attacker wants — and a broken filter must not be able to deny everyone either.

This deliberately does **not** reuse `EventSink.InvokeSocketConnect`: that fires later and allocates a `SocketConnectEventArgs` per connection, which is what the accept path avoids for rejected traffic.

## What ships behind it

**`firewall`** (UOContent) — the existing admin-curated set. Collapsed from `Firewall` + `AdminFirewall` + a threaded enforcer into one single-threaded store with **zero concurrency primitives**: the accept path, admin gump, TTL expiry and boot load all run on the game loop. Persists to `Configuration/firewall.json` with automatic migration from the legacy `firewall.cfg`. No behavior change for operators — same namespace, same gump, same commands.

**`blocklist`** (UOContent) — new. Holds a millions-strong list in-app and **demand-pages** hits up to CrowdSec, which promotes them to the OS firewall.

The motivation is concrete: CrowdSec's Windows bouncer cannot load the ~3.9M IPs that 91 community feeds produce, but it handles ~100k fine. So the millions live in-process behind a binary search, and only addresses that *actually connect* get promoted. A `PromotedGuard` suppresses re-reporting an address until the bouncer picks it up.

The list is parsed straight from UTF-8 file bytes with no per-line string allocation, off the game loop, and published as an immutable snapshot swapped through a single `volatile` reference. Reloads yield to world saves.

**`tools/Export-IpBlocklist.ps1`** — the producer. Requires PowerShell 7 and runs on Windows, Linux and macOS; Windows PowerShell 5.1 is refused up front via `#requires`. Merges a thin, non-overlapping feed set into one de-duplicated, bogon-filtered file. Parsing runs in a compiled `Add-Type` hot loop (~1s for ~4M lines instead of minutes). Written to a `.tmp` sibling and swapped with `File.Replace`, so the shard never reads a half-written list, and a total feed outage refuses to overwrite a good list with an empty one. Re-running is idempotent — it exits without downloading anything while the list on disk is younger than `-MinInterval` (default 2h, the anchor feed's own refresh period), so a misconfigured scheduler can't hammer upstream.

## CrowdSec: contribute-first

`IBanReporter` + `BanChannel` fan locally-decided bans out to external systems. `CrowdSecReporter` (UOContent) posts to LAPI `POST /v1/alerts` and retracts via `DELETE /v1/decisions`.

Reporting is **enqueue-only** on the accept path: a bounded, coalescing channel drained off-loop with bounded retry, counted drops on overflow, and a flush on shutdown. Under a DDoS the accept path never does synchronous or lock-contending per-IP work.

### Why not pull decisions from CrowdSec?

The original design streamed decisions into an in-app snapshot and enforced them at the accept gate. That's the wrong layer: by the time the shard sees the connection, the TCP handshake and socket setup are already paid for. `cs-firewall-bouncer` drops the same traffic **at the kernel**, and it's what CrowdSec is built to do. So the shard now contributes what it uniquely knows (rate-limit trips, blocklist hits from real connection attempts) and lets the OS enforce.

The one thing the OS can't do — hold millions of entries on Windows — is exactly what the in-app blocklist covers, and it feeds the same pipeline.

## Threading policy

`CLAUDE.md` rule #3 is rewritten as an explicit three-part policy, with rule #10 restated in tandem:

- Anything touching game state runs **only** on the main loop.
- Heavy work that *needs* game state must be **chunked** across ticks, never threaded.
- Heavy work that does *not* need game state (large-file parse, external I/O) **must** run off-loop **and must yield to world saves**.

Results come back via an immutable snapshot swapped through a single `volatile` reference, or `Core.LoopContext.Post` — never by letting the scheduler decide where heavy work runs. Both new subsystems follow it.

## Shared primitives

`SortedRangeIndex<T> where T : IBinaryInteger<T>` — coalesced disjoint interval arrays plus a binary search. The firewall, the blocklist, and (as of this PR) core's reserved-network tables all use it.

Coalescing is a correctness requirement, not an optimization: multi-feed lists nest CIDRs (`/24` containing a `/32`), and a search that inspects only the rightmost run whose minimum is ≤ the value is sound **only** over disjoint runs. That bug was caught in review and is covered by regression tests.

`IPAddressUtility` collects the allocation-free `IPAddress` ↔ `UInt128` conversions and CIDR parsing that were previously scattered or duplicated.

## Config

| File | Owner | Keys |
|---|---|---|
| `Configuration/bans.json` | core | `reportRateLimitTrips`, `autoBanDuration` |
| `Configuration/blocklist.json` | content | `file`, `reloadInterval`, `reportHits`, `banDuration`, `promoteSuppression` |
| `Configuration/crowdsec.json` | content | `lapiUrl`, `machineId`, `password`, `origin`, `manualBanDuration`, `flushInterval`, `maxQueue` |
| `Configuration/firewall.json` | content | persisted firewall entries (migrated from `firewall.cfg`) |

Everything is inert by default. CrowdSec self-disables without credentials; the blocklist self-disables until its file exists. A shard that changes nothing sees no behavior change.

## Notes for review

- **Core no longer references `Firewall` or `IFirewallEntry` anywhere.** `NetworkUtilities` used to build its reserved-network tables out of `CidrFirewallEntry`, which coupled core to the firewall for something unrelated to banning; those are now a `SortedRangeIndex<UInt128>`, same semantics and public API.
- **`BanChannel.Stop()` no longer persists the firewall** — a contribution coordinator has no business saving an enforcement store. That's the firewall filter's `Stop()`.
- **A dead `whitelisted` parameter was dropped** from the blocklist gate: it was hardcoded `false` at its only call site, and no whitelist concept exists in core.
- **The blocklist filter is an instance, not a static.** The static version forced its tests onto the sequential collection with a reset hook; they now run in parallel.
- `dev-docs/networking-packets.md` documents the seam for content authors, plus a known wart in the `IPAddress` ↔ `UInt128` normalization flagged for a follow-up PR.
- The generator was verified on Linux, macOS and Windows under a temporary CI matrix (since removed). It caught two portability bugs — a Windows-only path separator, and a culture-sensitive duration parse that read `2.5` as `25` on comma-decimal locales and *silently* turned a 2.5h cooldown into 25h — plus a third that made the script unparseable on Windows PowerShell 5.1. The source is ASCII-only for that last reason: `#requires` is only honored once a file parses, so non-ASCII in a BOM-less script produces parse errors instead of the version message.

## Tests

**1344 pass** (782 `Server.Tests`, 562 `UOContent.Tests`). New coverage: filter registry (registration, short-circuit, fault-disable), blocklist parsing/CIDR/coalescing, snapshot reload markers, promote-guard TTL, ban-channel fan-out, CrowdSec alert building/dedup/flush-on-stop, and the generator's output-format contract pinned against the reader.

Projects/UOContent/Misc/Firewall/BaseFirewallEntry.cs

@@ -0,0 +1,71 @@
+/*************************************************************************
+ * ModernUO                                                              *
+ * Copyright 2019-2026 - ModernUO Development Team                       *
+ * Email: hi@modernuo.com                                                *
+ * File: BaseFirewallEntry.cs                                            *
+ *                                                                       *
+ * This program is free software: you can redistribute it and/or modify  *
+ * it under the terms of the GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3 of the License, or     *
+ * (at your option) any later version.                                   *
+ *                                                                       *
+ * You should have received a copy of the GNU General Public License     *
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
+ *************************************************************************/
+
+using System;
+using System.Net;
+using System.Runtime.CompilerServices;
+
+namespace Server.Network;
+
+public abstract class BaseFirewallEntry : IFirewallEntry, ISpanFormattable
+{
+    public abstract UInt128 MinIpAddress { get; }
+    public abstract UInt128 MaxIpAddress { get; }
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public bool IsBlocked(IPAddress address) => IsBlocked(address.ToUInt128());
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public bool IsBlocked(UInt128 address) => address >= MinIpAddress && address <= MaxIpAddress;
+
+    public override string ToString() =>
+        MinIpAddress == MaxIpAddress ? MinIpAddress.ToIpAddress().ToString()
+            : $"{MinIpAddress.ToIpAddress()}-{MaxIpAddress.ToIpAddress()}";
+
+    public string ToString(string? format, IFormatProvider? formatProvider) =>
+        // format and provider are explicitly ignored
+        ToString();
+
+    public bool TryFormat(
+        Span<char> destination,
+        out int charsWritten,
+        ReadOnlySpan<char> format,
+        IFormatProvider? provider
+    )
+    {
+        if (!((ISpanFormattable)MinIpAddress.ToIpAddress()).TryFormat(destination, out charsWritten, format, provider))
+        {
+            return false;
+        }
+
+        if (MinIpAddress == MaxIpAddress)
+        {
+            return true;
+        }
+
+        // Range
+        destination[charsWritten++] = '-';
+
+        var total = charsWritten;
+
+        if (!((ISpanFormattable)MaxIpAddress.ToIpAddress()).TryFormat(destination[charsWritten..], out charsWritten, format, provider))
+        {
+            return false;
+        }
+
+        charsWritten += total;
+        return true;
+    }
+}