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/pipeline_state/apply.rs

@@ -98,20 +98,33 @@ pub fn apply_transition_str(
     apply_transition(story_id, event, content_transform).map_err(|e| e.to_string())
 }
 
-/// Freeze a story at its current stage.
+/// Freeze a story.
 ///
-/// Story 929: the YAML write of `resume_to_stage` is gone; the projection
-/// layer no longer reads it (defaults to Coding). Story 934 will make
-/// frozen a flag orthogonal to Stage, so the story stays in its current
-/// Stage rather than encoding a "where to resume" payload — at which point
-/// the read-side default also becomes moot.
-pub fn transition_to_frozen(story_id: &str) -> Result<TransitionFired, ApplyError> {
-    apply_transition(story_id, PipelineEvent::Freeze, None)
+/// Story 934, stage 4: `frozen` is now a CRDT flag orthogonal to [`Stage`],
+/// so the story stays at its current stage and only the boolean register
+/// changes.  Returns `Err(NotFound)` if no item exists for `story_id`.
+pub fn transition_to_frozen(story_id: &str) -> Result<(), ApplyError> {
+    if read_typed(story_id)?.is_none() {
+        return Err(ApplyError::NotFound(story_id.to_string()));
+    }
+    crate::crdt_state::set_frozen(story_id, true);
+    crate::slog!("[pipeline/transition] #{}: Freeze (flag set)", story_id);
+    Ok(())
 }
 
 /// Unfreeze a story.
 ///
-/// Story 929: paired with `transition_to_frozen`, no longer touches YAML.
-pub fn transition_to_unfrozen(story_id: &str) -> Result<TransitionFired, ApplyError> {
-    apply_transition(story_id, PipelineEvent::Unfreeze, None)
+/// Story 934, stage 4: paired with [`transition_to_frozen`]; clears the
+/// CRDT `frozen` flag without touching the stage register.  Returns
+/// `Err(NotFound)` if no item exists for `story_id`.
+pub fn transition_to_unfrozen(story_id: &str) -> Result<(), ApplyError> {
+    if read_typed(story_id)?.is_none() {
+        return Err(ApplyError::NotFound(story_id.to_string()));
+    }
+    crate::crdt_state::set_frozen(story_id, false);
+    crate::slog!(
+        "[pipeline/transition] #{}: Unfreeze (flag cleared)",
+        story_id
+    );
+    Ok(())
 }