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/crdt_state/mod.rs

@@ -52,11 +52,15 @@ pub use types::{
     TestJobCrdt, TestJobView, TokenUsageCrdt, TokenUsageView, WorkItem,
 };
 pub use write::{
-    bump_retry_count, migrate_names_from_slugs, migrate_story_ids_to_numeric, name_from_story_id,
-    set_agent, set_blocked, set_depends_on, set_epic, set_item_type, set_mergemaster_attempted,
-    set_name, set_qa_mode, set_retry_count, set_review_hold, write_item,
+    bump_retry_count, migrate_legacy_stage_strings, migrate_names_from_slugs,
+    migrate_story_ids_to_numeric, name_from_story_id, set_agent, set_blocked, set_depends_on,
+    set_epic, set_frozen, set_item_type, set_mergemaster_attempted, set_name, set_qa_mode,
+    set_retry_count, set_review_hold, write_item,
 };
 
+#[cfg(test)]
+pub use write::write_item_str;
+
 #[cfg(test)]
 pub use state::init_for_test;