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

crates/ledger-store/Cargo.toml

@@ -21,6 +21,11 @@ hex = "0.4"
 tempfile = "3"
 proptest = "1"
 rust_decimal = "1"
+criterion = "0.8.2"
+
+[[bench]]
+name = "store"
+harness = false
 
 [lints]
 workspace = true