[AZ-658] frame_ingest H.264/265 decoder (NVDEC + sw fallback)

Wires a real ffmpeg-next 8.1 decoder into the frame_ingest lifecycle
loop. NVDEC is probed at runtime via h264_cuvid / hevc_cuvid; CUDA-less
hosts transparently fall back to software h264 / hevc. Each decoded
frame is stamped with capture_ts (taken at packet receipt) and
decode_ts (taken after decode returns) so movement_detector sees
accurate frame-arrival times. Single-frame decode errors are counted
toward decode_errors_total and dropped; the stream is never aborted.

Adds new public API on FrameIngestHandle: decoder_backend(),
decode_errors_total(), frames_decoded_total(), decode_ms_first_frame(),
decode_ms_p50(), decode_ms_p99(). Integration tests under
crates/frame_ingest/tests/decoder_pipeline.rs cover AC-1, AC-3, AC-4
end-to-end through the real FfmpegDecoder using libx264-encoded
synthetic streams; AC-2 positive (NVDEC selection) is opt-in via
--ignored on a CUDA host. AZ-657 lifecycle tests retained via a
StubDecoder.

Co-authored-by: Cursor <cursoragent@cursor.com>

Cargo.toml

@@ -87,6 +87,11 @@ libc = "0.2"
 # Geospatial
 h3o = "0.7"
 
+# Multimedia (RTSP + H.264/265 decode for frame_ingest — see AZ-658).
+# Linked dynamically against the host FFmpeg 8.x install (libavcodec /
+# libavformat / libavutil / libswscale / libswresample) via pkg-config.
+ffmpeg-next = "8.1"
+
 # Test scaffolding
 wiremock = "0.6"
 tempfile = "3"