feat: ffprobe wrapper (tools.probe)

Per sub-project-2 plan Task 3 / spec §5.1. Parses ffprobe JSON into a Probe
(streams + format); subprocess runner injectable so tests never shell out.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tools/probe.py

@@ -0,0 +1,34 @@
+"""ffprobe wrapper -> parsed streams/format. Subprocess runner is injectable."""
+
+from __future__ import annotations
+
+import json
+import subprocess
+from dataclasses import dataclass
+
+
+@dataclass
+class Probe:
+    streams: list
+    format: dict
+
+
+def _default_runner(args) -> str:
+    return subprocess.run(args, capture_output=True, text=True, check=True).stdout
+
+
+def probe_file(path, *, runner=_default_runner) -> Probe:
+    out = runner(
+        [
+            "ffprobe",
+            "-v",
+            "quiet",
+            "-print_format",
+            "json",
+            "-show_format",
+            "-show_streams",
+            str(path),
+        ]
+    )
+    data = json.loads(out)
+    return Probe(streams=data.get("streams", []), format=data.get("format", {}))