server/world/chunk: fix unbounded loop in SubChunk.Layer and layer renumbering in compact

[schphe]

Scope

Neither of these is reachable from a packet, a world save or in-game play. Both are reachable from the package's own exported API. Filing them as hardening, not as live bugs.

The loop

SubChunk.Layer compared uint8(len(sub.storages)) against the layer index. At 256 storages that narrowing wraps to 0, which is <= every uint8, so the loop never terminated and appended a 4096-entry storage until the process ran out of memory. Layer(255) on a fresh sub chunk is enough; layer 254 terminates.

Every SetBlock call in dragonfly passes a literal layer of 0 or 1, and the decode path never calls Layer at all, so only a caller outside the package can reach it.

A sub chunk's storage count is encoded as a single byte in both the network and disk formats, which also caps a decoded sub chunk at 255 storages. A 256th cannot be represented at all, encoding one would write a count of 0 followed by 256 storages of payload. Layer now rejects layers at or above the new MaxLayers rather than building a sub chunk that cannot be written out. The three other places that narrowed a length to compare against a layer index are compared as ints for the same reason.

The renumbering

SubChunk.compact dropped every all-air storage and closed the gap, moving each surviving layer down. Layer numbers are semantic, layer 0 is the block, layer 1 is Bedrock's waterlogging layer, so a sub chunk with air in layer 0 and water in layer 1 came out with the water in layer 0.

Waterlogging always writes a block to layer 0, and removing that block promotes the liquid back to layer 0, so a chunk built through Tx never holds that state. It is reachable through SetOpts.DisableLiquidDisplacement and through a Structure returning a liquid with no block. Only trailing all-air storages carry no information, so only those are dropped now.

Verification

Regression tests fail without the fix: Layer(255) never returns within 5s, and Compact() block at layer 0 = 1, want 0 (air): it was renumbered down from layer 7. A randomised test over 30 seeds also asserts compaction changes no block at any layer, that one was written after a round-trip fuzz rediscovered the renumbering independently.