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/startup/project.rs

@@ -151,6 +151,12 @@ pub(crate) async fn init_subsystems(app_state: &Arc<SessionState>, cwd: &Path) {
             crate::slog!("[crdt] Failed to initialise CRDT state layer: {e}");
         } else {
             crdt_state::migrate_names_from_slugs();
+            // Story 934 stage 6: rewrite any pre-934 directory-style stage
+            // strings to the clean post-934 wire vocabulary, and set the new
+            // `frozen` flag on items that were previously at `Stage::Frozen`.
+            // Must run before legacy stage-string acceptance is dropped from
+            // `Stage::from_dir` (also part of stage 6).
+            crdt_state::migrate_legacy_stage_strings();
             let id_migrations = crdt_state::migrate_story_ids_to_numeric();
             if !id_migrations.is_empty()
                 && let Some(project_root) = db_path.parent().and_then(|p| p.parent())