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/runtime/src/lib.rs

@@ -84,9 +84,9 @@ pub use hooks::{
 };
 pub use lane_events::{
     compute_event_fingerprint, dedupe_superseded_commit_events, dedupe_terminal_events,
-    is_terminal_event, EventProvenance, LaneCommitProvenance, LaneEvent, LaneEventBlocker,
-    LaneEventBuilder, LaneEventMetadata, LaneEventName, LaneEventStatus, LaneFailureClass,
-    LaneOwnership, SessionIdentity, WatcherAction,
+    is_terminal_event, BlockedSubphase, EventProvenance, LaneCommitProvenance, LaneEvent,
+    LaneEventBlocker, LaneEventBuilder, LaneEventMetadata, LaneEventName, LaneEventStatus,
+    LaneFailureClass, LaneOwnership, SessionIdentity, WatcherAction,
 };
 pub use mcp::{
     mcp_server_signature, mcp_tool_name, mcp_tool_prefix, normalize_name_for_mcp,