fix(protocol): fix multi-version client protocol compatibility

[Q2297045667]

Multi-version protocol compatibility fixes

Improved multi-version protocol compatibility and stability for legacy (1.7–1.12) Java clients.

Root Cause

1. Container Content Packet (CSetContainerContent)

• Offhand slot: The player inventory screen (window ID 0) always contains 46 slots, where index 45 is the offhand slot. The offhand slot was introduced in 1.9, so 1.7/1.8 clients only expect 45 slots. Receiving 46 slots made those clients crash with IndexOutOfBoundsException: Index: 45, Size: 45 while running Container.getSlot(45).
• Fix: When version < 1.9 && window_id == 0 && slot_data.len() == 46, skip index 45 (the offhand slot) and write a slot count of 45. Behavior for 1.9+ is unchanged. Serialization is read-only, so the server-side offhand stack is never mutated across client-version switches.

2. Multi Block Update Packet (CMultiBlockUpdate)

• Multi-version chunk handling: Previously the server always wrote the 1.13+ format (i64 chunk position + VarLong records) for every version. 1.9–1.12 clients expect two separate chunk X/Z ints and, per record, a u16 position followed by a VarInt block state; 1.7/1.8 use different formats again. The mismatch caused 1.9–1.12 clients to crash mid-decode with readerIndex ... exceeds writerIndex.
• Fix: Write the correct format per target version:
  • >= 1.13: i64 chunk section + VarInt count + VarLong records (unchanged).
  • 1.9 – 1.12: chunkX (i32) + chunkZ (i32) + VarInt count + (u16 position + VarInt block state).
  • 1.8: chunkX/chunkZ + VarInt count + (u8 horizontal position + u8 y + VarInt block state).
  • 1.7.x: chunkX/chunkZ + short count + i32 data length + (u16 position + u16 block state), where block state is (block id << 4) | metadata.

Testing

• Added unit tests covering offhand-slot stripping, cross-version offhand persistence, and every MultiBlockChange version boundary (1.7 / 1.8 / 1.9 / 1.13+).
• cargo test -p pumpkin-protocol --lib → 105 passed, 0 failed.
• cargo check --all-targets --all-features, cargo clippy --all-targets --all-features, and cargo fmt --all -- --check all pass.
• cargo-machete reports no unused dependencies.

Known remaining issues

• 1.13 entity spawn: SPacketSpawnMob / SPacketSpawnObject still send legacy 1.12 numeric entity IDs instead of 1.13 registry IDs (the ViaBackwards mappings-1.13to1.12.nbt has no entities section), causing EntitySmallFireball cannot be cast to EntityLivingBase on 1.13 clients.
• 1.16+ / 1.19+ join: not yet diagnosed; client logs show no Java exception.

Related PRs

• fix(entity): show burning animation on older client versions #3078
• fix(entity): send metadata to clients on older protocol versions #3066
• Fix(protocol) 1.21.x to 26.1 for: Oxygen, Villagers, Boats, Commands and more. #3055
• fix(worldgen): existing chunk state loading #3050
• fix(player): change MAIN_ARM_ID metadata to ARM type for 1.21.11 #2954
• Fixes command packet disconnecting 1.21.4 clients. #2884
• Client version control #2614