Previously decay() wrote all nodes to the append log on every run,
even if their weight was unchanged (factor of 1.0 or negligible
delta). Now only nodes with meaningful weight change get version
bumped and persisted.
Also simplified: near-prune clamping now happens inline instead of
in a separate pass.
mod.rs was 937 lines with all Store methods in one block.
Split into three files by responsibility:
- persist.rs (318 lines): load, save, replay, append, snapshot
— all disk IO and cache management
- ops.rs (300 lines): upsert, delete, modify, mark_used/wrong,
decay, fix_categories, cap_degree — all mutations
- mod.rs (356 lines): re-exports, key resolution, ingestion,
rendering, search — read-only operations
No behavioral changes; cargo check + full smoke test pass.
