unseen: configurable pool size and slice count (--pool-size / --slices)

The draw hardcoded 300 repos / 3 slices. At the measured lazy-fetch rate
the full 300 projects ~2.5 days; a 100-repo single slice (slice 1, all
the G0 baseline needs) is ~20h. Add --pool-size (default 300) and
--slices (default 3), validated (>=1, slices<=pool_size, evenly
divisible). Generalize assign_slices' stratified allocation from a
literal 3 to `slices`, the run() break / insufficient check / report
(changes = pool_size*13) to pool_size, and record both in the journal
inputs. Remove bisect's three-slice assumption; freeze reads the size
from the draft. Defaults (300/3) reproduce prior output exactly.

With --pool-size 100 --slices 1 the pool is the first 100 accepted repos
in seed order, all in slice 1 (deterministic, byte-identical across
worker counts). 280 unseen tests pass, ruff clean.

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

tests/test_unseen_bisect.py

@@ -14,7 +14,7 @@ d = importlib.import_module("benchmarks.unseen.draft")
 b = importlib.import_module("benchmarks.unseen.bisect_guard")
 
 
-def draft_pair(tmp_path, count=9):
+def draft_pair(tmp_path, count=9, slices=3):
     root, cache = tmp_path / "tree", tmp_path / "cache"
     root.mkdir()
     cache.mkdir()
@@ -29,7 +29,7 @@ def draft_pair(tmp_path, count=9):
                 "url": "https://" + f"github.com/synthetic/candidate-{i}",
                 "license_class": "private" if i % 2 else "permissive",
                 "license_spdx": "GPL-3.0" if i % 2 else "MIT",
-                "slice": i % 3 + 1,
+                "slice": i % slices + 1,
                 "changes": changes,
             }
         )