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/db/mod.rs

@@ -328,7 +328,7 @@ mod tests {
         write_item_with_content(story_id, "2_current", content, meta);
 
         let view = crate::crdt_state::read_item(story_id).expect("story exists in CRDT");
-        assert_eq!(view.stage_str(), "2_current");
+        assert_eq!(view.stage().as_dir(), "coding");
         assert_eq!(view.name(), Some("Typed Name"));
         assert_eq!(view.agent(), Some("coder-1"));
         assert_eq!(view.retry_count(), 2);
@@ -353,7 +353,7 @@ mod tests {
         write_item_with_content(story_id, "2_current", content, ItemMeta::default());
 
         let view = crate::crdt_state::read_item(story_id).expect("story exists in CRDT");
-        assert_eq!(view.stage_str(), "2_current");
+        assert_eq!(view.stage().as_dir(), "coding");
         assert_eq!(
             view.name(),
             None,
@@ -406,7 +406,7 @@ mod tests {
 
         // Seed the story in 2_current with retry_count = 3 (a coder that
         // burned all its retries).
-        crate::crdt_state::write_item(
+        crate::crdt_state::write_item_str(
             story_id,
             "2_current",
             Some("Retry reset test"),
@@ -433,7 +433,7 @@ mod tests {
         let typed_after = crate::pipeline_state::read_typed(story_id)
             .expect("read should succeed")
             .expect("story exists in CRDT");
-        assert_eq!(typed_after.stage.dir_name(), "4_merge");
+        assert_eq!(typed_after.stage.dir_name(), "merge");
         assert_eq!(
             typed_after.retry_count, 0,
             "retry_count must reset to 0 on stage transition"