unseen: implement freeze p1 (group 3: bisect guard, freeze writer, protocol)

Group 3 was unimplemented, so there was no way to turn the draft into a
frozen pool. Add:

- bisect_guard.py: stages the cache draft and bisects it against the
  reserved-repository guard, run as a black box
  (pytest -k referenced, stdout/stderr discarded, exit code only; an exit
  other than 0/1 aborts as an instrument failure). Drops guard-tripping
  candidates and reports the dropped COUNT only -- never a name or URL.
  Guard runner is injectable for offline tests.
- freeze.py: writes canonical pool-p1.toml (+ private counterpart,
  reusing draft's public/private split and private_digest) and
  freeze-p1.json (used-set digest, per-source counts, guard result,
  dropped count, label-check counts, freeze digest = sha256 of the
  canonical public manifest). A matching retry is a no-op; any changed
  frozen artifact requires a new pool id (never an overwrite).
- protocol-p1.md: the frozen scoring/statistics protocol from design
  sections 3 and 4.

Offline only; the guard subprocess is the sole subprocess and is mocked
in tests. 42 new tests pass (incl the group-1.3 guard digest check on a
fixture pool); neighbouring unseen suites green; ruff clean. New files
only -- does not touch the journal-pinned draft/extract modules, so a
running draw is unaffected.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

.kiro/specs/unseen-repo-evaluation/tasks.md

@@ -130,14 +130,14 @@ Execution rules:
   - _Requirements: 2.1, 2.2_
 
 - [ ] 3. Freeze p1 (design 1.2, 6; increment 3)
-- [ ] 3.1 Guard bisection tool
+- [x] 3.1 Guard bisection tool
   - `benchmarks/unseen/bisect_guard.py`: rewrites the draft with a candidate subset, runs
     `.venv/bin/pytest -q -p no:cacheprovider tests/test_independent_population.py -k referenced` with stdout and
     stderr discarded, reads only the exit code, and bisects until green. Reports the dropped count only.
   - `tests/test_unseen_bisect.py` with an injected fake guard (a hidden set): finds and drops exactly the hidden
     members; never logs a candidate; an exit code other than 0 or 1 aborts (instrument failure).
   - _Requirements: 1.1_
-- [ ] 3.2 Freeze writer
+- [x] 3.2 Freeze writer
   - `benchmarks/unseen/freeze.py`: canonical `pool-p1.toml` (and the private counterpart) from the bisected draft;
     `freeze-p1.json` with used-set digest and per-source counts, guard result, dropped count, label-check counts
     and the freeze digest (sha256 of the canonical manifest). Refuses to write over a frozen pool; a correction
@@ -145,7 +145,7 @@ Execution rules:
   - Tests: digest stable under key order; overwrite refused; the guard's digest test (1.3) passes on a fixture
     pool.
   - _Requirements: 1.2_
-- [ ] 3.3 Protocol p1
+- [x] 3.3 Protocol p1
   - `benchmarks/unseen/protocol-p1.md`: the scoring rules of design section 4 (default flags, decision 4.0's
     long-deadline arm, catch and location rules, false alarm, coverage states), the statistics of section 3 and
     7, and the adoption gate of 7.3, stated before any replay.