Implement LaneEvent schema extensions for event ordering, provenance, and dedupe (US-002)

Adds comprehensive metadata support to LaneEvent for the canonical lane event schema:

- EventProvenance enum: live_lane, test, healthcheck, replay, transport
- SessionIdentity: title, workspace, purpose, with placeholder support
- LaneOwnership: owner, workflow_scope, watcher_action (Act/Observe/Ignore)
- LaneEventMetadata: seq, provenance, session_identity, ownership, nudge_id,
  event_fingerprint, timestamp_ms
- LaneEventBuilder: fluent API for constructing events with full metadata
- is_terminal_event(): detects Finished, Failed, Superseded, Closed, Merged
- compute_event_fingerprint(): deterministic fingerprint for terminal events
- dedupe_terminal_events(): suppresses duplicate terminal events by fingerprint

Provides machine-readable event provenance, session identity at creation,
monotonic sequence ordering, nudge deduplication, and terminal event suppression.

Adds 10 regression tests covering:
- Monotonic sequence ordering
- Provenance serialization round-trip
- Session identity completeness
- Ownership and workflow scope binding
- Watcher action variants
- Terminal event detection
- Fingerprint determinism and uniqueness
- Terminal event deduplication
- Builder construction with metadata
- Metadata serialization round-trip

Closes Phase 2 (partial) from ROADMAP.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Yeachan-Heo