feat(ltk-overlay): binary game index

[Crauzer]

No description provided.

[Copilot]

Pull request overview

This pull request optimizes the game index caching system by switching from JSON to MessagePack serialization and changing content hash computation from eager pre-computation to lazy on-demand calculation.

Changes:

• Replaced JSON serialization with MessagePack for the game index cache (file renamed from game_index_cache.json to game_index.bin)
• Removed pre-computed content hashes from GameIndex structure to avoid hashing ~500k chunks during indexing
• Added on-demand compute_content_hashes_batch() method that only computes hashes for chunks actually overridden by mods
• Refactored index building to perform a single directory walk, reusing results across multiple builder functions

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
crates/ltk_overlay/src/game_index.rs	Converted cache serialization to MessagePack, removed content_hashes field, added on-demand batch content hash computation, refactored to reuse directory walk results
crates/ltk_overlay/src/builder.rs	Updated filter_overrides to use new on-demand content hash computation, updated documentation references from .json to .bin
crates/ltk_overlay/Cargo.toml	Added rmp-serde dependency for MessagePack serialization
Cargo.lock	Reflected rmp-serde dependency addition

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