fix: serialize chat run state mutations

[chubes4]

Summary

• add a generic optional atomic mutation seam for run-control stores and use it for chat ownership, lifecycle, enqueue, and claim mutations
• implement the default mutex with bounded non-autoloaded rows in existing WordPress options storage, including token expiry, conditional takeover, transactional fencing, and token-checked release
• reserve run IDs before handlers execute, including pre-session dispatch, so foreign-owner and same-owner duplicate requests cannot both run
• preserve canonical conversation-session ownership, workspace scoping, single-site compatibility, and generic layer purity
• add no custom production tables or migrations

Concurrency Coverage

• independent MariaDB connections serialize site and workspace state mutations
• expired holders cannot overwrite takeover winners
• different-owner, same-owner, and pre-session duplicate run races admit one execution
• double claims return each queued message exactly once
• enqueue concurrent with claim loses or resurrects no messages
• active contention, stale takeover, exception release, corrupt state, cache coherence, and single-site fallback

Tests

• composer test
• composer phpstan
• composer validate --strict
• php tests/agents-chat-ability-smoke.php
• php tests/default-agents-chat-handler-smoke.php
• php tests/provider-turn-adapter-smoke.php
• php tests/chat-run-control-smoke.php
• php tests/chat-run-control-multisite-smoke.php
• php tests/bootstrap-version-skew-smoke.php
• real WordPress wpdb / MariaDB integration: php tests/run-control-option-store-integration.php with isolated option and sitemeta fixtures
• PHP syntax checks and git diff --check

Follow-up to #452 and #451.