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/read.rs

@@ -363,6 +363,11 @@ pub(super) fn extract_item_view(item: &PipelineItemCrdt) -> Option<PipelineItemV
         _ => None,
     };
 
+    let frozen = match item.frozen.view() {
+        JsonValue::Bool(b) => Some(b),
+        _ => None,
+    };
+
     Some(PipelineItemView {
         story_id,
         stage,
@@ -379,6 +384,7 @@ pub(super) fn extract_item_view(item: &PipelineItemCrdt) -> Option<PipelineItemV
         review_hold,
         item_type,
         epic,
+        frozen,
     })
 }
 
@@ -458,7 +464,7 @@ mod tests {
     use super::super::state::init_for_test;
     use super::super::state::rebuild_index;
     use super::super::types::PipelineItemCrdt;
-    use super::super::write::write_item;
+    use super::super::write::write_item_str;
     use super::*;
     use bft_json_crdt::json_crdt::OpState;
     use bft_json_crdt::keypair::make_keypair;
@@ -540,7 +546,7 @@ mod tests {
         let story_id = "917_story_concurrent_evict";
 
         // Insert the story locally (simulates node 1's insert).
-        write_item(
+        write_item_str(
             story_id,
             "1_backlog",
             Some("Node 1 insert"),
@@ -611,7 +617,7 @@ mod tests {
         );
 
         // story_id must be in the tombstone set so write_item cannot resurrect it.
-        write_item(
+        write_item_str(
             story_id,
             "1_backlog",
             Some("Resurrection attempt"),