Validate frame-accurate seeking with python-mpv (build order step 1)

Confirms single-frame stepping is reliable: monotonic frame counter
across steps, and the synthetic clip's 1-frame flash event lands on
exactly one frame with clean neighbors. Also documents/works around
three environment quirks hit along the way: locally-built libmpv not
in ldconfig's cache, QApplication resetting LC_NUMERIC and breaking
libmpv's locale requirement, and the playback-restart event not firing
reliably after frame-step (switched to polling estimated_frame_number).

scripts/mpv_bare_test.py

@@ -0,0 +1,34 @@
+import locale
+locale.setlocale(locale.LC_NUMERIC, "C")
+import mpv
+import time
+from pathlib import Path
+
+CLIP = str(Path(__file__).resolve().parent.parent / "test_media" / "sync_test_INSYNC.mp4")
+
+m = mpv.MPV(hr_seek="yes", hr_seek_framedrop="no", keep_open="yes", pause=True, vo="null")
+
+events_seen = []
+@m.event_callback('playback-restart', 'seek', 'file-loaded')
+def cb(evt):
+    events_seen.append(str(evt))
+
+m.play(CLIP)
+m.wait_for_property("duration", timeout=10)
+m.pause = True
+print("loaded, duration=", m.duration, "fps=", m.container_fps)
+
+m.seek(4.9, reference="absolute", precision="exact")
+time.sleep(0.5)
+print("after seek: time_pos=", m.time_pos, "frame=", m.estimated_frame_number)
+
+for i in range(5):
+    before = m.time_pos
+    try:
+        with m.prepare_and_wait_for_event('playback-restart', timeout=2.0):
+            m.command('frame-step')
+        print(f"step {i}: OK via event, time_pos={m.time_pos} frame={m.estimated_frame_number}")
+    except TimeoutError:
+        print(f"step {i}: TIMEOUT waiting for playback-restart, time_pos={m.time_pos} frame={m.estimated_frame_number} (before={before})")
+
+m.terminate()