feat(pathfinding): .swb format v8 compact index (#3b) (#2471)

## Summary
Phase #3b (final roadmap item), stacked on #2470. Compacts the index trailer from 20 to 8 bytes/chunk.

Trammel: 19.2 MB → 17.9 MB. Roadmap total: 565 MB → 17.9 MB (−96.8%).

## Details
- Trailer stores `{ u32 packedKey = (ChunkX << 16) | ChunkY, u32 recordLength }` per chunk, in record write order; the file offset is dropped and reconstructed by cumulative recordLength from HeaderSize.
- No record reordering, no varint; fixed-stride, TryReadChunk unchanged.
- Also simplifies the accumulated `.swb` code comments across the stack.
- Format v8; v7 files rejected and re-baked once.

## Tests
v8 multi-chunk round-trip (cumulative offset reconstruction) + the v6/v7 suite; full pathfinding suite green; Release build clean.

Projects/UOContent/Engines/Pathing/BitmapAStarAlgorithm.cs

@@ -16,6 +16,7 @@
 using System;
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
+using Server.Engines.Pathing;
 using Server.Engines.Pathing.Cache;
 using Server.Mobiles;
 using Server.Systems.FeatureFlags;
@@ -114,7 +115,7 @@ public class BitmapAStarAlgorithm : PathAlgorithm
         // and trips the threshold immediately.
         StepCache.Instance.BeginFindGeneration();
 
-        Server.Engines.Pathing.PathfindRecorder.RecordIfEnabled(m, map, start, goal);
+        PathfindRecorder.RecordIfEnabled(m, map, start, goal);
 
         _currentMobileNeedsSlowPath = RequiresSlowPath(m);
         _currentMobilePlayerStrict = m.Player && m.AccessLevel < AccessLevel.GameMaster;
@@ -548,6 +549,5 @@ public class BitmapAStarAlgorithm : PathAlgorithm
     /// + wetMask). CanOpenDoors / CanMoveOverObstacles only affect dynamic items and don't
     /// disqualify the cache.
     /// </summary>
-    private static bool RequiresSlowPath(Mobile m) =>
-        m is BaseCreature bc && bc.CanFly;
+    private static bool RequiresSlowPath(Mobile m) => m is BaseCreature bc && bc.CanFly;
 }