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.Tests/Tests/Engines/Pathing/StepCacheFileV7Tests.cs

@@ -7,9 +7,7 @@ namespace Server.Tests.Pathfinding;
 
 // v7 = per-chunk libdeflate compression on top of the v6 predictive-Z format. Each record is
 // compressed independently (random access preserved) behind a u32 uncompressed-length prefix;
-// records that do not shrink (tiny Uniform records) are stored raw. StepCacheFileV6Tests already
-// round-trips every chunk shape through this compression path; these tests cover the v7-specific
-// behavior: that compression engages, that the stored-raw fallback round-trips, and v6 rejection.
+// records that do not shrink (tiny Uniform records) are stored raw.
 [Collection("Sequential Pathfinding Tests")]
 public class StepCacheFileV7Tests
 {