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/PathCacheCommands.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics;
 using System.IO;
 using Server.Engines.Pathing.Cache;
 
@@ -95,12 +96,12 @@ public static class PathCacheCommands
 
         var totalChunks = 0;
         var totalMaps = 0;
-        var sw = System.Diagnostics.Stopwatch.StartNew();
+        var sw = Stopwatch.StartNew();
 
         for (var i = 0; i < Map.Maps.Length; i++)
         {
             var map = Map.Maps[i];
-            if (map == null || map == Map.Internal || (only.HasValue && map.MapID != only.Value))
+            if (map == null || map == Map.Internal || only.HasValue && map.MapID != only.Value)
             {
                 continue;
             }