test(core): add comprehensive tests for lock.rs and store.rs
Day 1 improvement cycle: reinforce test coverage for deterministic
task protocol primitives.
lock.rs: +6 tests (acquire unknown task, wrong owner renew, release
noop, no-conflict check, multi-file lock/release, empty sweep)
store.rs: +6 tests (replay since_id, empty replay, upsert replace,
remove task, legacy file loading, lock release event rebuild,
lease_expiry calculation)
Closes #87
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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