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/work_item/freeze.rs

@@ -28,10 +28,8 @@ pub enum UnfreezeStatus {
 /// stage without making any CRDT writes.  Returns `Err` if the state transition
 /// fails (e.g. the item is not found or is in a terminal stage).
 pub fn freeze(story_id: &str) -> Result<FreezeStatus, String> {
-    let already_frozen = crate::pipeline_state::read_typed(story_id)
-        .ok()
-        .flatten()
-        .map(|i| i.stage.is_frozen())
+    let already_frozen = crate::crdt_state::read_item(story_id)
+        .map(|view| view.frozen())
         .unwrap_or(false);
 
     if already_frozen {
@@ -45,13 +43,11 @@ pub fn freeze(story_id: &str) -> Result<FreezeStatus, String> {
 
 /// Unfreeze a work item, resuming normal pipeline behaviour.
 ///
-/// Returns [`UnfreezeStatus::NotFrozen`] if the item is not currently in the
-/// frozen stage.  Returns `Err` if the state transition fails.
+/// Returns [`UnfreezeStatus::NotFrozen`] if the item is not currently frozen.
+/// Returns `Err` if the state transition fails.
 pub fn unfreeze(story_id: &str) -> Result<UnfreezeStatus, String> {
-    let is_frozen = crate::pipeline_state::read_typed(story_id)
-        .ok()
-        .flatten()
-        .map(|i| i.stage.is_frozen())
+    let is_frozen = crate::crdt_state::read_item(story_id)
+        .map(|view| view.frozen())
         .unwrap_or(false);
 
     if !is_frozen {
@@ -92,7 +88,7 @@ mod tests {
             .expect("read_typed should succeed")
             .expect("item should be present");
         assert!(
-            item.stage.is_frozen(),
+            item.is_frozen(),
             "stage should be Frozen after freeze: {:?}",
             item.stage
         );
@@ -141,7 +137,7 @@ mod tests {
             .expect("read_typed should succeed")
             .expect("item should be present");
         assert!(
-            !item.stage.is_frozen(),
+            !item.is_frozen(),
             "stage should not be Frozen after unfreeze: {:?}",
             item.stage
         );
@@ -212,12 +208,12 @@ mod tests {
             .expect("MCP-path item should be in CRDT");
 
         assert!(
-            state_a.stage.is_frozen(),
+            state_a.is_frozen(),
             "chat-path CRDT stage must be frozen: {:?}",
             state_a.stage
         );
         assert!(
-            state_b.stage.is_frozen(),
+            state_b.is_frozen(),
             "MCP-path CRDT stage must be frozen: {:?}",
             state_b.stage
         );