Adopt pinned public core snapshot and prepare Sized 2.0.0

scripts/check-core.py

@@ -0,0 +1,14 @@
+#!/usr/bin/env python3
+"""Verify the exact public core snapshot without contacting its private forge."""
+import hashlib
+import json
+from pathlib import Path
+
+root = Path(__file__).resolve().parent.parent
+pin = json.loads((root / 'CORE-SNAPSHOT.json').read_text())
+snapshot = root / 'crates/sized-core'
+actual = {str(p.relative_to(snapshot)): hashlib.sha256(p.read_bytes()).hexdigest()
+          for p in snapshot.rglob('*') if p.is_file()}
+if actual != pin['files']:
+    raise SystemExit('Core snapshot differs from CORE-SNAPSHOT.json; review the upstream pin and every changed file.')
+print(f"Core snapshot verified: {pin['revision']} ({len(actual)} files)")