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/http/mcp/story_tools/epic.rs

@@ -126,16 +126,20 @@ pub(crate) fn tool_show_epic(args: &Value, _ctx: &AppContext) -> Result<String,
             continue;
         };
         if member_view.epic() == Some(epic_id) {
-            let stage_name = match &item.stage {
-                Stage::Upcoming | Stage::Backlog => "backlog",
-                Stage::Coding => "current",
-                Stage::Qa => "qa",
-                Stage::Merge { .. } => "merge",
-                Stage::Done { .. } => "done",
-                Stage::Archived { .. } => "archived",
-                Stage::MergeFailure { .. } => "merge_failure",
-                Stage::Frozen { .. } => "frozen",
-                Stage::Blocked { .. } => "blocked",
+            // Frozen is now an orthogonal CRDT flag (story 934, stage 4).
+            let stage_name = if member_view.frozen() {
+                "frozen"
+            } else {
+                match &item.stage {
+                    Stage::Upcoming | Stage::Backlog => "backlog",
+                    Stage::Coding => "current",
+                    Stage::Qa => "qa",
+                    Stage::Merge { .. } => "merge",
+                    Stage::Done { .. } => "done",
+                    Stage::Archived { .. } => "archived",
+                    Stage::MergeFailure { .. } => "merge_failure",
+                    Stage::Blocked { .. } => "blocked",
+                }
             };
             member_items.push(json!({
                 "story_id": sid,