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/MoveResult.cs

@@ -1,12 +1,11 @@
-namespace Server
-{
-    public delegate MoveResult MoveMethod(Direction d, bool badStateOk);
+namespace Server;
 
-    public enum MoveResult
-    {
-        BadState,
-        Blocked,
-        Success,
-        SuccessAutoTurn
-    }
-}
+public delegate MoveResult MoveMethod(Direction d, bool badStateOk);
+
+public enum MoveResult
+{
+    BadState,
+    Blocked,
+    Success,
+    SuccessAutoTurn
+}