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

Cargo.lock

@@ -3564,6 +3564,7 @@ name = "ledger-store"
 version = "0.1.0"
 dependencies = [
  "chrono",
+ "criterion",
  "hex",
  "ledger-core",
  "proptest",