Strengthen event log WAL semantics

[shunichironomura]

Summary

• add durable event-log append/create helpers with file and parent-directory syncs
• serialize record-action transactions with per-log advisory lock files
• move action persistence into an ExecutionStore boundary to keep I/O code contained
• create executions via a staged durable directory then atomic rename
• copy, verify, and sync attachments before committing AttachmentAdded events
• add non-mutating core event builders so actions build/validate events before durable append

Verification

• just check-all
• just test

Note: just check-all currently reports an existing frontend lint warning for src/lib/index.ts being empty, but exits successfully.