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/Server/Compression/Deflate.cs

@@ -28,8 +28,6 @@ public static class Deflate
 
     public static LibDeflateBinding Standard => _standard ??= new LibDeflateBinding();
 
-    // Best-ratio compressor. Construction allocates a native libdeflate compressor, so it is
-    // cached per thread like Standard and reused. Decompression is level-independent, so the
-    // decompress path can use either accessor. libdeflate is not thread-safe — hence ThreadStatic.
+    // Best-ratio compressor, cached per thread like Standard (construction allocates native state).
     public static LibDeflateBinding Maximum => _maximum ??= new LibDeflateBinding(LibDeflateCompressionLevel.VeryHigh);
 }