feat(pathfinding): .swb uniform-chunk elision (format v5) — Trammel 592→232 MB (#2465)

## Summary

Sub-project #1 of the `.swb` step-cache size-reduction roadmap (`dev-docs/pathfinding.md` § Future work). Adds **uniform-chunk elision** to the `StepCacheFile` format, bumping it **v4 → v5**.

A fully-uniform 16×16 chunk — no strata, **no swim layer**, all 19 base arrays constant (open ocean, Green Acres, void) — serializes to a **~28-byte record** (`KindUniform`) instead of ~5,393, and reconstructs **byte-identically** via `Array.Fill`. Non-uniform chunks use the existing v4 body (`KindFull`) with the swim-layer and strata trailers **fully preserved** — the Kind byte is just prepended.

## Calibrated result (measured, not projected)

Baked Trammel via `SaveToFile`:

| | |
|---|---:|
| Chunks | 114,688 |
| Uniform (swim-aware) → elided | 62.7% |
| Swim-layer chunks (stay Full) | 8.9% |
| Strata chunks (stay Full) | 1.8% |
| Baseline (full records) | 592.2 MB |
| **Actual v5 `.swb`** | **231.9 MB (−61%)** |

The residual is ~150 MB of non-uniform land Z-blocks (targeted by #2 predictive-Z) + ~81 MB of swim-layer trailers (#2/#3). #2 and #3 are separate follow-up PRs.

## Implementation

- `StepChunk.IsUniform()` — false if it has strata **or a swim layer**, else true only when all 19 base arrays are constant (the "all-same" check uses the SIMD-accelerated `ContainsAnyExcept`).
- `StepCacheFile` v5 — `Kind` byte (`KindFull=0`/`KindUniform=2`, 1 reserved); uniform write/read; `FormatVersion`/`MinSupportedVersion` → 5 (v4 files rejected on open and re-baked). No `StepCache`/algorithm/index changes; fingerprint logic untouched.

## Tests

7 `StepCacheFileV5Tests` (uniform round-trip + `<200 B` compactness, varied-full, swim-layer-full, strata-full, swim+strata combined, v4 version-gate rejection) + the existing StepCache/pathfinding suite — **70 pass**, including the prior `SwimLayer_RoundTrips`. An independent review verified write/read symmetry, cast round-tripping, swim/strata preservation, and the version gate (READY TO MERGE).

Projects/UOContent/Engines/Pathing/Cache/StepChunk.cs

@@ -174,4 +174,29 @@ internal sealed class StepChunk
 
     /// <summary>Serialization hook: returns the raw data array (or null if no strata).</summary>
     internal byte[] GetStrataDataForSerialization() => _strataData;
+
+    /// <summary>
+    /// True when every cell shares one value across WalkMask, WetMask, SourceZ, and all 16
+    /// directional-Z arrays, and the chunk has neither multi-Z strata nor a swim layer. Such a
+    /// chunk serializes to a ~28-byte uniform record (StepCacheFile v5) instead of the full
+    /// record. Chunks with a swim layer (shore cells) are never uniform — their per-cell swim
+    /// data must be preserved via the Full record.
+    /// </summary>
+    internal bool IsUniform()
+    {
+        if (_strataOffsetByCell != null || HasSwimLayer)
+        {
+            return false;
+        }
+        return AllSame(WalkMask) && AllSame(WetMask) && AllSame(SourceZ)
+               && AllSame(WalkZN) && AllSame(WalkZNE) && AllSame(WalkZE) && AllSame(WalkZSE)
+               && AllSame(WalkZS) && AllSame(WalkZSW) && AllSame(WalkZW) && AllSame(WalkZNW)
+               && AllSame(SwimZN) && AllSame(SwimZNE) && AllSame(SwimZE) && AllSame(SwimZSE)
+               && AllSame(SwimZS) && AllSame(SwimZSW) && AllSame(SwimZW) && AllSame(SwimZNW);
+    }
+
+    // "All 256 cells equal" via SIMD-accelerated ContainsAnyExcept (skip cell 0, the reference).
+    private static bool AllSame(byte[] a) => a.Length < 2 || !a.AsSpan(1).ContainsAnyExcept(a[0]);
+
+    private static bool AllSame(sbyte[] a) => a.Length < 2 || !a.AsSpan(1).ContainsAnyExcept(a[0]);
 }