fix(cli): make dispatch storage locks portable

[limityan]

Summary

• replace the Windows no-op Dispatch locks with the workspace's existing cross-platform file-lock implementation
• recover incomplete event-log tails through read/write handles and an explicit append position
• keep retention non-blocking, skip active jobs, and retry Windows sharing violations on a later collection pass
• make the process-identity cancellation test platform-independent without adding Windows detached-worker support

Root cause

The Windows lock helper reported success without acquiring a lock. Event logs were opened in append mode even though recovery needs truncation, retention attempted to rename a directory while its lock file was still open, and one cancellation test depended on Unix-only process behavior.

Scope

• no storage schema, lock path, wire contract, or public API changes
• no Windows detached-worker implementation
• no new runtime or process-management abstraction

Verification

• cargo test -q -p bitfun-cli (476 unit tests plus all CLI integration suites)
• cargo check --workspace
• node scripts/check-core-boundaries.mjs
• node scripts/check-repo-hygiene.mjs
• git diff HEAD^ --check
• rustfmt --edition 2021 --check src/apps/cli/src/dispatch/mod.rs src/apps/cli/src/dispatch/store.rs