[TEST-05] `state` crate has 90 unit tests but zero integration / persistence / crash-recovery tests

[intendednull]

Commit: 2f26d91 · Finding: TEST-05

Problem

crates/state has 190 #[test] items inline, but no crates/state/tests/ integration directory. In-module tests use in-memory doubles and cannot detect serialization drift, schema-migration bugs, or partial-write recovery. The absence of tests/ also means the public API of willow-state is never exercised against the real storage backend, nor across restart() boundaries.

There is no end-to-end "state + storage survives crash" test anywhere in the workspace.

Fix

Add crates/state/tests/persistence.rs driving the public API against the real storage backend:

• Normal write → reopen → read round-trip.
• Mid-write kill (use tempfile + crash-simulating writer) → reopen → re-apply pending events.
• Schema migration bump scenarios.

Obvious? Moderate — the setup is mechanical but writing the crash-simulator takes care.