feat: pool manifest read/write with atomic save

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

gates/pool.py

@@ -0,0 +1,49 @@
+"""Pure storage layer for the Image Pool node. Stdlib only — no torch, no comfy."""
+import json
+import os
+from pathlib import Path
+
+
+def empty_manifest():
+    return {"active": 0, "slots": [], "next_seq": 1}
+
+
+def pool_dir(base_dir, pool_id):
+    return Path(base_dir) / pool_id
+
+
+def manifest_path(base_dir, pool_id):
+    return pool_dir(base_dir, pool_id) / "manifest.json"
+
+
+def read_manifest(base_dir, pool_id):
+    p = manifest_path(base_dir, pool_id)
+    if not p.exists():
+        return empty_manifest()
+    try:
+        with open(p, "r", encoding="utf-8") as f:
+            m = json.load(f)
+        # minimal shape guard
+        if not isinstance(m, dict) or "slots" not in m:
+            raise ValueError("bad manifest")
+        m.setdefault("active", 0)
+        m.setdefault("next_seq", len(m.get("slots", [])) + 1)
+        return m
+    except (ValueError, json.JSONDecodeError):
+        return rebuild_manifest(base_dir, pool_id)
+
+
+def write_manifest(base_dir, pool_id, manifest):
+    d = pool_dir(base_dir, pool_id)
+    d.mkdir(parents=True, exist_ok=True)
+    final = d / "manifest.json"
+    tmp = d / "manifest.json.tmp"
+    with open(tmp, "w", encoding="utf-8") as f:
+        json.dump(manifest, f, indent=2)
+    os.replace(tmp, final)  # atomic on same filesystem
+    return manifest
+
+
+def rebuild_manifest(base_dir, pool_id):
+    # Temporary stub — replaced in Task 7.
+    return empty_manifest()