feat: pool manifest read/write with atomic save

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_pool.py

@@ -0,0 +1,28 @@
+import json
+from pathlib import Path
+from gates import pool
+
+
+def test_empty_manifest_shape():
+    m = pool.empty_manifest()
+    assert m == {"active": 0, "slots": [], "next_seq": 1}
+
+
+def test_read_missing_creates_empty(tmp_path):
+    m = pool.read_manifest(str(tmp_path), "p1")
+    assert m == pool.empty_manifest()
+
+
+def test_write_then_read_roundtrip(tmp_path):
+    m = pool.empty_manifest()
+    m["active"] = 2
+    pool.write_manifest(str(tmp_path), "p1", m)
+    # file lives at <base>/p1/manifest.json
+    assert (tmp_path / "p1" / "manifest.json").exists()
+    assert pool.read_manifest(str(tmp_path), "p1") == m
+
+
+def test_write_is_atomic_no_partial_temp_left(tmp_path):
+    pool.write_manifest(str(tmp_path), "p1", pool.empty_manifest())
+    leftovers = list((tmp_path / "p1").glob("*.tmp"))
+    assert leftovers == []