feat(local-coord-mcp): durability module — append log + replay

Adds cartridges/local-coord-mcp/ffi/coord_durability.zig. Append-only
event log with CRC32-trailed records, replay-on-init, and typed log
helpers for the five coord tables (inbox, claims, audit, quarantine
overflow, track record).

Feature-gated on BOJ_COORD_STATE_DIR — unset leaves every entry point
a silent no-op, so existing tests and in-memory-only deployments are
unaffected. Future task replaces the backend with verisimdb-mcp's FFI
once it's real; the typed log helpers are the stable seam.

5 round-trip tests pass (empty-env no-op, open-creates-dir, single-event
round-trip, CRC-truncation tolerance, all-14-event-types decode).

Task #7 Pass B, step 1/3. Wiring into local_coord_ffi.zig next.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath