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>
dave
Timmy