feat(934): typed Stage enum replaces directory-string state model

The state machine's `Stage` enum becomes the source of truth for pipeline
state. Six stages of work land together:

  1. Clean wire vocabulary (`coding`, `merge`, `merge_failure`, ...) replaces
     legacy directory-style strings (`2_current`, `4_merge`, ...) on the wire.
     `Stage::from_dir` accepted both during deployment; new writes always
     emit the clean form via `stage_dir_name`. Lexicographic `dir >= "5_done"`
     checks in lifecycle.rs become typed `matches!` checks since the new
     vocabulary doesn't sort in pipeline order.
  2. `crdt_state::write_item` takes typed `&Stage`, serialising via
     `stage_dir_name` at the CRDT boundary. `#[cfg(test)] write_item_str`
     parses legacy strings for test fixtures.
  3. `WorkItem::stage()` returns typed `crdt_state::Stage`; `stage_str()`
     is gone from the public API. Projection dispatches on the typed enum.
  4. `frozen` becomes an orthogonal CRDT register. `Stage::Frozen` and
     `PipelineEvent::Freeze`/`Unfreeze` are removed; `transition_to_frozen`/
     `unfrozen` set the flag directly without touching the stage register.
  5. Watcher sweep and `tool_update_story`'s `blocked` setter route through
     `apply_transition` so the typed transition table validates every
     stage change. `update_story` gains a `frozen` field for symmetry.
  6. One-shot startup migration rewrites pre-934 directory-style stage
     registers (and sets `frozen=true` on items previously at `7_frozen`).
     `Stage::from_dir` drops legacy aliases. The db boundary keeps a small
     normaliser so callers with legacy strings (MCP, tests) still work.

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

server/src/service/status/format.rs

@@ -83,8 +83,8 @@ mod tests {
         let event = StatusEvent::StageTransition {
             story_id: "42_story_foo".to_string(),
             story_name: Some("Foo Story".to_string()),
-            from_stage: "4_merge".to_string(),
-            to_stage: "5_done".to_string(),
+            from_stage: "merge".to_string(),
+            to_stage: "done".to_string(),
         };
         let s = format_status_event(&event);
         assert!(
@@ -101,8 +101,8 @@ mod tests {
         let event = StatusEvent::StageTransition {
             story_id: "10_story_bar".to_string(),
             story_name: Some("Bar".to_string()),
-            from_stage: "1_backlog".to_string(),
-            to_stage: "2_current".to_string(),
+            from_stage: "backlog".to_string(),
+            to_stage: "coding".to_string(),
         };
         let s = format_status_event(&event);
         assert!(!s.contains("\u{1f389}"));
@@ -114,8 +114,8 @@ mod tests {
         let event = StatusEvent::StageTransition {
             story_id: "5_story_x".to_string(),
             story_name: None,
-            from_stage: "2_current".to_string(),
-            to_stage: "3_qa".to_string(),
+            from_stage: "coding".to_string(),
+            to_stage: "qa".to_string(),
         };
         let s = format_status_event(&event);
         assert!(s.contains("5_story_x"));
@@ -182,8 +182,8 @@ mod tests {
             StatusEvent::StageTransition {
                 story_id: "1_story_a".to_string(),
                 story_name: None,
-                from_stage: "1_backlog".to_string(),
-                to_stage: "2_current".to_string(),
+                from_stage: "backlog".to_string(),
+                to_stage: "coding".to_string(),
             },
             StatusEvent::MergeFailure {
                 story_id: "2_story_b".to_string(),