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/story/front_matter.rs

@@ -4,32 +4,19 @@
 //! without performing any I/O. Parsing is delegated to `crate::io::story_metadata`.
 
 #[allow(dead_code)]
-/// Return `true` if `stage` is a recognised pipeline stage directory name.
+/// Return `true` if `stage` is a recognised pipeline stage name.
 ///
-/// Valid stage names match the `.huskies/work/N_name/` directory scheme.
+/// Accepts both the clean post-934 wire form (e.g. `"backlog"`) and the
+/// legacy directory-style form (e.g. `"1_backlog"`).
 pub fn is_valid_stage(stage: &str) -> bool {
     crate::pipeline_state::Stage::from_dir(stage).is_some()
 }
 
 #[allow(dead_code)]
-/// Map a human-readable stage alias (e.g. `"backlog"`) to its directory name
-/// (e.g. `"1_backlog"`). Returns `None` for unrecognised aliases.
+/// Map any recognised stage alias (clean wire form or legacy directory form)
+/// to the canonical clean wire form. Returns `None` for unrecognised aliases.
 pub fn stage_alias_to_dir(alias: &str) -> Option<&'static str> {
-    use crate::pipeline_state::Stage;
-    // Canonical directory names (e.g. "1_backlog") round-trip through the typed enum.
-    if let Some(stage) = Stage::from_dir(alias) {
-        return Some(stage.dir_name());
-    }
-    // Short human-readable aliases (user-facing input normalization).
-    match alias {
-        "backlog" => Some("1_backlog"),
-        "current" => Some("2_current"),
-        "qa" => Some("3_qa"),
-        "merge" => Some("4_merge"),
-        "done" => Some("5_done"),
-        "archived" => Some("6_archived"),
-        _ => None,
-    }
+    crate::pipeline_state::Stage::from_dir(alias).map(|s| s.dir_name())
 }
 
 // ── Tests ─────────────────────────────────────────────────────────────────────
@@ -40,36 +27,42 @@ mod tests {
 
     #[test]
     fn is_valid_stage_accepts_all_known_stages() {
-        assert!(is_valid_stage("1_backlog"));
-        assert!(is_valid_stage("2_current"));
-        assert!(is_valid_stage("3_qa"));
-        assert!(is_valid_stage("4_merge"));
-        assert!(is_valid_stage("5_done"));
-        assert!(is_valid_stage("6_archived"));
+        // Clean post-934 vocabulary.
+        assert!(is_valid_stage("backlog"));
+        assert!(is_valid_stage("coding"));
+        assert!(is_valid_stage("qa"));
+        assert!(is_valid_stage("merge"));
+        assert!(is_valid_stage("done"));
+        assert!(is_valid_stage("archived"));
     }
 
     #[test]
     fn is_valid_stage_rejects_unknown() {
-        assert!(!is_valid_stage("current"));
-        assert!(!is_valid_stage("backlog"));
+        // Story 934 stage 6 dropped legacy directory-style aliases.
+        assert!(!is_valid_stage("current")); // pre-934 short alias, no longer mapped
+        assert!(!is_valid_stage("1_backlog"));
+        assert!(!is_valid_stage("2_current"));
         assert!(!is_valid_stage("7_future"));
         assert!(!is_valid_stage(""));
     }
 
     #[test]
     fn stage_alias_maps_short_names() {
-        assert_eq!(stage_alias_to_dir("backlog"), Some("1_backlog"));
-        assert_eq!(stage_alias_to_dir("current"), Some("2_current"));
-        assert_eq!(stage_alias_to_dir("qa"), Some("3_qa"));
-        assert_eq!(stage_alias_to_dir("merge"), Some("4_merge"));
-        assert_eq!(stage_alias_to_dir("done"), Some("5_done"));
-        assert_eq!(stage_alias_to_dir("archived"), Some("6_archived"));
+        assert_eq!(stage_alias_to_dir("backlog"), Some("backlog"));
+        assert_eq!(stage_alias_to_dir("coding"), Some("coding"));
+        assert_eq!(stage_alias_to_dir("qa"), Some("qa"));
+        assert_eq!(stage_alias_to_dir("merge"), Some("merge"));
+        assert_eq!(stage_alias_to_dir("done"), Some("done"));
+        assert_eq!(stage_alias_to_dir("archived"), Some("archived"));
     }
 
     #[test]
-    fn stage_alias_maps_full_dir_names() {
-        assert_eq!(stage_alias_to_dir("1_backlog"), Some("1_backlog"));
-        assert_eq!(stage_alias_to_dir("6_archived"), Some("6_archived"));
+    fn stage_alias_returns_none_for_legacy_dir_names() {
+        // Story 934 stage 6: legacy directory-style aliases are no longer
+        // recognised — startup migration rewrites stored CRDT values, and
+        // user-facing aliases now use only the clean wire vocabulary.
+        assert_eq!(stage_alias_to_dir("1_backlog"), None);
+        assert_eq!(stage_alias_to_dir("6_archived"), None);
     }
 
     #[test]