fix(manager): stamp event ids with the run identity readers validate

[Huang-404-Q]

What this fixes

With a custom --log-dir (anything outside <runs-root>/<run-id>/lh_harness), the dashboard's event feed stays empty for a healthy run: every record is silently dropped with event_id does not belong to the requested run.

Why: the worker stamps event ids from the event log's grandparent path component (manager.py _append_event):

run_id = path.parents[2].name if len(path.parents) > 2 else "local"

but the dashboard/API validates each id against the run id it derives — the reserved id under --runs-root, or log_dir.parent.name for a pinned --log-dir. Those only coincide for the default layout. Concretely, lh-harness run --log-dir /x/myrun --dashboard stamps ids myrun:000001… while the embedded dashboard reads with the reserved id 20260828T000000Z_abcd1234 — the strict identity guard (webapi/events.py) then rejects every record, so REST replay, WebSocket deltas, and the snapshot's last-200 events all come back empty. The standalone web --log-dir path hits the same wall through its "local" fallback.

The change

• HarnessConfig gains an optional run_id — the reader-facing run identity.
• _append_event accepts run_event_id and prefers it over the path-derived stamp; every manager call site passes config.run_id.
• The CLI threads the reserved run id into the config. Without a configured id (direct/test callers), the legacy path-derived stamp is kept, so default-layout behavior is unchanged.
• The cross-run identity guard itself is untouched: foreign prefixes are still rejected.

Tests

tests/test_event_run_id.py (3 tests):

• a configured run_id overrides the path-derived stamp;
• without one, the stamp still falls back to the log directory's parent (default layout unchanged);
• the reader accepts the configured stamp and keeps rejecting foreign ones.

Red/green verified: the first and third fail on main (empty feed for a pinned --log-dir), all pass with the fix. Full suite: 408 passed, 1 skipped; ruff clean on the touched files.