store: benchmarks at 10k entries on disk

benches/store.rs (criterion, in `just bench`): open 249 ms, viewer load
616 ms, full verify 809 ms, post one entry (four synced appends) 62 ms.
The books are built on a temp dir and copied under target/ so every
sync hits a real disk. Baselines in docs/testing.md; no budget, by
decision.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX

justfile

@@ -69,6 +69,7 @@ mutants:
 # Benchmarks of the core's hot paths (no budget gates them yet)
 bench:
     cargo bench -p ledger-core --bench books
+    cargo bench -p ledger-store --bench store
 
 # Build the release binaries (CLI, MCP server, GUI)
 build: