ROADMAP #133: Blocked-state subphase contract — implement §6.5

Adds BlockedSubphase enum with 7 variants for structured blocked-state reporting:
- blocked.trust_prompt — trust gate blockers
- blocked.prompt_delivery — prompt misdelivery
- blocked.plugin_init — plugin startup failures
- blocked.mcp_handshake — MCP connection issues
- blocked.branch_freshness — stale branch blockers
- blocked.test_hang — test timeout/hang
- blocked.report_pending — report generation stuck

LaneEventBlocker now carries optional subphase field that gets serialized
into LaneEvent data. Enables clawhip to route recovery without pane scraping.

Updates:
- lane_events.rs: BlockedSubphase enum, LaneEventBlocker.subphase field
- lane_events.rs: blocked()/failed() constructors with subphase serialization
- lib.rs: Export BlockedSubphase
- tools/src/lib.rs: classify_lane_blocker() with subphase: None
- Test imports and fixtures updated

Backward-compatible: subphase is Option<>, existing events continue to work.

rust/crates/tools/src/lib.rs

@@ -4459,6 +4459,7 @@ fn classify_lane_blocker(error: &str) -> LaneEventBlocker {
     LaneEventBlocker {
         failure_class: classify_lane_failure(error),
         detail,
+        subphase: None,
     }
 }