fix: Removes unnecessary dictionary removal guards (#2565)

## What

`Dictionary<K,V>.Remove` and `HashSet<T>.Remove` do not bump the collection's version, so removing an entry during a `foreach` does not invalidate the enumerator. A number of loops were still paying for a `PooledRefQueue`/`PooledRefList` to collect keys and drain them in a second pass. This drops those guards.

## Why it's safe

Verified against .NET 10.0.10 rather than taken on trust, since the documented guarantee covers only `Dictionary<TKey,TValue>.Remove` while several of these call sites are `HashSet<T>` or enumerate `.Keys`/`.Values`:

| Case | Result |
|---|---|
| `Dictionary` foreach + `Remove` | safe, all entries visited |
| `Dictionary.Keys` / `.Values` foreach + `Remove` | safe, all entries visited |
| `HashSet` foreach + `Remove` | safe, all entries visited |
| `Dictionary` foreach + `Remove` **then `Add`** | throws `InvalidOperationException` |

Reflection on `_version` confirms the mechanism: neither `Dictionary.Remove` nor `HashSet.Remove` touches it. Because `Remove` never bumps the version, the `Keys` and `Values` enumerators are just as safe as the dictionary's own, even though only `Dictionary.Remove` documents the behaviour. No entries were skipped in any case.

The `HashSet` half is confirmed by [stephentoub on dotnet/dotnet-api-docs#8177](https://github.com/dotnet/dotnet-api-docs/issues/8177#issuecomment-1167251052): *"Both HashSet and Dictionary have been improved to support removal during enumeration. The docs may just benefit from updating."* The gap is in the documentation, not the runtime.

`Remove` followed by `Add` in the same enumeration still throws. That is the line this PR does not cross.

## Guards removed

`VisibilityList`, `ChampionTitleSystem`, `Channel`, `BombingRun`, `Ruleset`, `PuzzleChest`, `RaceChangeGump`, `StepCache`, `PlayerMurderSystem`, `VirtueSystem`, `ProjectedItem`, `StaminaSystem`, `AIGroupMovement`, `PromotedGuard`, `AutoDenylist`, `LoginAllowlist`, `AntiMacroSystem`, `DetectHidden`.

Both collection kinds are covered: `Dictionary` (including loops over `.Keys` and `.Values`) and `HashSet` (`ProjectedItem._active`, `PlayerMurderSystem._contextTerms`, `StaminaSystem._resetHash`). In `StaminaSystem.ResetTimer` the `Count == queue.Count → Clear()` branch goes away with the queue — it only existed to avoid paying for N individual removes.

Where the collection supports it, `Contains` + `Remove` and `TryGetValue` + `Remove` also collapse into a single lookup (`if (list.Remove(x))`, `if (m_Pending.Remove(ns, out var state))`).

`Utility.Tidy<K,V>` keeps its two branches: when `K` is serializable the value is not inspected, otherwise the value is. Only the serializable side may be cast, so `Dictionary<Mobile, int>` and `Dictionary<Mobile, string>` stay valid.

## Deliberately unchanged

**`BaseCreature.LoyaltyTimer.OnTick`** keeps its deferred-delete queue. Removing from `World.Mobiles` while enumerating it is safe, but `Mobile.Delete()` is not a `Remove` — it runs `OnDelete`/`OnAfterDelete`, the `OnParentDeleted` cascade over the creature's pack, `DropHolding()`, and region and guild callbacks. Anything in that surface that constructs a `Mobile` is an `Add` into the dictionary being enumerated, which does invalidate it. `BaseHire.PayTimer.OnTick` has the same shape and is likewise untouched.

**Spatial-query buffers** — `GuardedRegion.CallGuards`, `Thunderstorm`, `Exorcism`, `LeverPuzzleController`, `BaseCreature.TeleportPets` — are a different hazard. They buffer the result of a range query because the drain moves or harms mobiles, which mutates sectors mid-enumeration.

**Re-entrant drains.** The `_users` sets in `Firebomb` and the explosion, conflagration and confusion-blast potions look like this pattern but are not: the loop collects, `Clear()`s, and only then runs `Target.Cancel` on each, which can re-enter. `AnimalTrainer` enumerates `pm.Stabled` and drains through `RemoveStabled`, which nulls the `Stabled` field once it empties — safe for an in-flight enumerator, which holds the set reference rather than the field, but subtle enough not to be worth inlining on a cold path.

## Verification

`dotnet build` clean with 0 warnings; 810 Server and 684 UOContent tests pass.

Projects/UOContent/Network/LoginAllowlist/LoginAllowlist.cs

@@ -20,7 +20,6 @@ using System.IO;
 using System.Net;
 using System.Text;
 using System.Threading.Tasks;
-using Server.Collections;
 using Server.Logging;
 using Server.Network.Bans;
 
@@ -230,13 +229,15 @@ public static class LoginAllowlist
         var stamps = new long[_allowed.Count];
         var count = 0;
 
-        using var expired = new PooledRefList<UInt128>(16);
+        var dropped = 0;
 
         foreach (var (address, stamp) in _allowed)
         {
             if (stamp < cutoff)
             {
-                expired.Add(address);
+                _allowed.Remove(address);
+                _strikes.Remove(address);
+                dropped++;
                 continue;
             }
 
@@ -245,19 +246,12 @@ public static class LoginAllowlist
             count++;
         }
 
-        for (var i = 0; i < expired.Count; i++)
-        {
-            _allowed.Remove(expired[i]);
-            _strikes.Remove(expired[i]);
-        }
-
         PruneStaleStrikes(nowUnix);
 
         _dirty = false;
 
         var path = _path;
         var total = count;
-        var dropped = expired.Count;
 
         _ = Task.Run(() => Write(path, addresses, stamps, total, dropped));
     }
@@ -270,20 +264,13 @@ public static class LoginAllowlist
             return;
         }
 
-        using var stale = new PooledRefList<UInt128>(16);
-
         foreach (var (address, strike) in _strikes)
         {
             if (nowUnix - strike.WindowStart > _strikeWindowSeconds)
             {
-                stale.Add(address);
+                _strikes.Remove(address);
             }
         }
-
-        for (var i = 0; i < stale.Count; i++)
-        {
-            _strikes.Remove(stale[i]);
-        }
     }
 
     private static void Write(string path, UInt128[] addresses, long[] stamps, int count, int dropped)