feat: add headless libretro core probe

core_probe.py loads a libretro core with no window, runs it for a
number of frames and reports the files it opened (strace), its log
lines, frame, audio and save-state hashes and its speed. bench repeats
a run after warm-up and gives median, mean, deviation and range; check
swaps one system file between runs and compares one observable.

--record writes a JSON record with the core build, frontend commit,
host and every input by hash, validated against
core-probe-record.schema.json. A busy host is refused, and a
repetition that disagrees with the first is flagged.

scripts/validate_schemas.py

@@ -616,6 +616,12 @@ def main() -> int:
     errors.extend(_save_directory_blocks())
     errors.extend(_registry_contract())
     errors.extend(_event_schema_errors())
+    errors.extend(
+        _validate_json_files(
+            sorted((ROOT / "measurements").glob("**/*.json")),
+            "core-probe-record.schema.json",
+        )
+    )
 
     if not args.source_only:
         database_path = ROOT / "database.json"