fix(opl): refuse property list invalidation raised from inside GetProperties (#2555)

## The bug

Any property getter reached from `GetProperties` that calls `InvalidateProperties` takes the tooltip build down with it:

```
System.ArgumentNullException: Value cannot be null. (Parameter 'array')
   at Server.ObjectPropertyList.AppendStringDirect(String value)
   at Server.Mobiles.PlayerMobile.GetProperties(IPropertyList list)
```

`InvalidateProperties` rebuilds **in place** — `Reset()`, then `GetProperties()` again on the same instance — and `Reset()` does two destructive things to a build already in flight:

1. **It returns the pooled interpolation buffer.** The compiler rents it in the handler ctor and returns it in the closing `Add`, so *every hole is evaluated while it is live*:

```csharp
var handler = new InterpolatedStringHandler(1, 2, list); // InitializeInterpolation() RENTS
handler.AppendFormatted(pl.Rank.Title);                  // <-- getter runs HERE
handler.AppendLiteral("\t");
handler.AppendFormatted(faction.Definition.PropName);
list.Add(1060776, ref handler);                          // consumes span, RETURNS
```

```
GetProperties(list)
├─ InitializeInterpolation()  -> _arrayToReturnToPool = Rent(256)     buffer LIVE
├─ « hole 1: pl.Rank.Title »
│  └─ PlayerState.Rank.get   (lazy recompute)
│     └─ Invalidate() -> InvalidateProperties() -> m_PropertyList.Reset()
│        └─ Dispose(): Return(buf); _arrayToReturnToPool = null        buffer GONE
└─ handler.AppendFormatted("Knight")
   └─ _arrayToReturnToPool.AsSpan(_pos..)
      └─ ArgumentNullException (Parameter 'array')
```

It surfaces as `ArgumentNullException` rather than `NullReferenceException` because the `Range` overload of `AsSpan` must read `array.Length`, so the BCL null-checks and names the parameter `array`.

2. **It rewinds the packet cursor**, so properties already written are overwritten by the nested pass — a silently corrupted tooltip even where the buffer survives.

## The fix: refuse, don't recover

There is no correct recovery, and retrying the build would only hide the defect. A nested invalidation now logs an error with a stack trace, **throws in `DEBUG`** so it gets found and fixed, and in `RELEASE` returns without touching the list — a possibly stale tooltip, but no crash, no corrupted packet, and nothing leaked back to the pool. Getters that genuinely must invalidate should defer:

```csharp
Timer.DelayCall(InvalidateProperties);
```

The guard flag lives on the `ObjectPropertyList`, not the entity: it is that list's own lifecycle, it costs nothing (both `Item` and `ObjectPropertyList` absorb it in existing padding, and the list is allocated lazily), and it stays correct when builds for different entities nest.

Base instance sizes are unchanged from `main`: Item 128 B, Mobile 792 B, ObjectPropertyList 72 B, PlayerMobile 1216 B.

`PropertyList` also publishes the list into `m_PropertyList` **before** building it rather than assigning through `??=` afterwards, so a nested `InvalidateProperties` sees the build in progress instead of recursing into a second throwaway list whose work is discarded.

`ObjectPropertyList` re-rents its scratch buffer instead of spanning a null array, so a stray `Reset()` from any other caller degrades rather than aborting `GetProperties`.

## Factions `PlayerState`: maintained, not lazily computed

The getter that surfaced this is now a plain field read — the whole `if (m_InvalidateRank)` block and the flag itself are gone:

```csharp
public RankDefinition Rank => m_Rank;
```

`UpdateRank()` recomputes at each point an input actually changes:

| Site | Why |
|---|---|
| `RankIndex` setter | this player's index changed |
| end of `KillPoints` setter | two paths write `m_RankIndex` directly, bypassing the setter; runs once the swap bookkeeping and `ZeroRankOffset` have settled |
| `Faction.AddMember` | *after* the insert — the member count is not settled during the ctor |
| `FactionState` load | once ordering and `ZeroRankOffset` are final |

Supporting fixes this forced out:

- **Both ctors seed the lowest rank.** Nothing recomputes on read any more, so `Rank` has to be usable immediately — including for members that never get a `RankIndex` assigned, which is *every member with no kill points*. Without this, `Rank.Title` NREs.
- **`Rank` always resolves.** Ranks are ordered by `Required` descending ending at `0`, so a *negative* percent (`RankIndex` out of sync with `ZeroRankOffset`) matched nothing and left `m_Rank` null. It no longer divides by a zero `ZeroRankOffset` either.
- **A pre-existing staleness bug.** The `KillPoints` setter writes `m_RankIndex` directly in two places, so the cached rank was never refreshed when a player crossed zero kill points.

All six readers of `Rank` were checked; none relied on the old side effect.

One behaviour change worth flagging: rank refreshes are now **eager** where they used to be lazy, so a `KillPoints` change invalidates each swapped player as it happens. The swap loops break as soon as ordering is satisfied — typically 0–2 swaps — but it is on the path that runs on every faction kill.

## Documentation

The rule is written down so it is enforceable rather than folklore:

- **CLAUDE.md** audit rule 19
- **`dev-docs/property-lists.md`** — new "Never Invalidate From Inside `GetProperties`" section with the failing/passing pattern
- **`dev-docs/claude-skills/modernuo-property-lists.md`** — key rule + anti-pattern
- **`dev-docs/claude-skills/modernuo-code-audit.md`** — rule 19, ERROR severity

## Tests

- `ObjectPropertyListReentrancyTests` — `Reset()` and `Dispose()` re-entered mid-hole (both red against `main` with the exact exception above), nesting behaviour, and the new contract: `DEBUG` throws, `RELEASE` survives, and the build is never retried into a loop.
- `FactionRankTests` — `Rank` is populated before anything reads it, tracks `RankIndex` without a read, is stable across reads, and still resolves when `RankIndex` is out of sync with `ZeroRankOffset`. Red-verified: removing the ctor seed fails the first one.

793/793 `Server.Tests` and 608/608 `UOContent.Tests` pass.

## Noted, not addressed here

`~ObjectPropertyList()` returns the rented array to `STArrayPool<char>.Shared` from the **finalizer thread**, and that pool is single-threaded by design. Left alone as a separate concern.

Projects/UOContent.Tests/Tests/Engines/Factions/FactionRankTests.cs

@@ -0,0 +1,100 @@
+using System.Collections.Generic;
+using Server;
+using Server.Factions;
+using Xunit;
+
+namespace UOContent.Tests;
+
+/// <summary>
+/// PlayerState.Rank is read from GetProperties, so it must stay a plain field read. These pin what
+/// that requires: the rank is never null, and it is correct without anyone having read it first.
+/// </summary>
+[Collection("Sequential UOContent Tests")]
+public class FactionRankTests
+{
+    // The faction ctor builds its own Definition, so no world state is needed.
+    private static Faction NewFaction() => new CouncilOfMages();
+
+    private static PlayerState AddMember(Faction faction, List<PlayerState> owner)
+    {
+        var state = new PlayerState(new Mobile(), faction, owner);
+        owner.Add(state);
+        return state;
+    }
+
+    [Fact]
+    public void Rank_IsPopulatedBeforeAnythingReadsIt()
+    {
+        var faction = NewFaction();
+        var state = new PlayerState(new Mobile(), faction, []);
+
+        // Nothing recomputes on read, so the ctor must leave a usable value or Rank.Title NREs.
+        Assert.NotNull(state.Rank);
+        Assert.NotNull(state.Rank.Title);
+    }
+
+    [Fact]
+    public void Rank_IsTheLowestRank_ForAnUnrankedMember()
+    {
+        var faction = NewFaction();
+        var owner = new List<PlayerState>();
+        var a = AddMember(faction, owner);
+        var b = AddMember(faction, owner);
+
+        a.UpdateRank();
+        b.UpdateRank();
+
+        var lowest = faction.Definition.Ranks[^1];
+
+        Assert.Equal(lowest.Rank, a.Rank.Rank);
+        Assert.Equal(lowest.Rank, b.Rank.Rank);
+    }
+
+    [Fact]
+    public void SettingRankIndex_UpdatesRankWithoutAnyoneReadingIt()
+    {
+        var faction = NewFaction();
+        var owner = new List<PlayerState>();
+        var top = AddMember(faction, owner);
+        var bottom = AddMember(faction, owner);
+
+        faction.ZeroRankOffset = 2;
+
+        top.RankIndex = 0;
+        bottom.RankIndex = 1;
+
+        // No read triggered these, yet the ordering is reflected.
+        Assert.True(top.Rank.Rank > bottom.Rank.Rank);
+    }
+
+    [Fact]
+    public void ReadingRank_IsStableAndSideEffectFree()
+    {
+        var faction = NewFaction();
+        var owner = new List<PlayerState>();
+        var state = AddMember(faction, owner);
+
+        faction.ZeroRankOffset = 1;
+        state.RankIndex = 0;
+
+        var first = state.Rank;
+        var second = state.Rank;
+
+        Assert.Same(first, second);
+    }
+
+    [Fact]
+    public void RankIndexOutOfSyncWithZeroRankOffset_StillResolvesARank()
+    {
+        var faction = NewFaction();
+        var owner = new List<PlayerState>();
+        var a = AddMember(faction, owner);
+        AddMember(faction, owner);
+
+        // A negative percent used to match no rank at all, leaving Rank null.
+        faction.ZeroRankOffset = 1;
+        a.RankIndex = 5;
+
+        Assert.NotNull(a.Rank);
+    }
+}