feat(encode): add Jetson Multimedia API encoder backend

Integrate a native Jetson Multimedia API encoder path and keep the existing encoded access-unit contract for RTMP, RTP, and recording consumers.

This adds conditional Jetson MMAPI detection in CMake, builds the required NVIDIA sample common classes into a dedicated support library, and compiles the new backend only when the platform dependencies are present.

The runtime selector now lets encoder.backend=auto prefer Jetson MM for NVIDIA hardware requests while keeping encoder.backend=ffmpeg as an explicit FFmpeg path. Device selection semantics are updated so nvidia requires the Jetson backend, auto can fall back to FFmpeg software, and software remains FFmpeg-only.

The Jetson backend converts supported raw inputs through swscale, feeds NvVideoEncoder in YUV420M, emits Annex-B access units, and harvests decoder configuration from a warmup keyframe so downstream packetizers keep their existing contract.

This also splits FFmpeg encoder option handling into a shared header, updates runtime config/help text and tester defaults, and refreshes compatibility/caveat documentation to reflect the new selection behavior.

Build-tree runtime RPATH handling is tightened so GCC 15 builds keep the matching libstdc++ visible locally. Verification covered GCC 15 builds, RTP H.264/H.265 tester runs, RTMP H.264 stub publish, local live SRS smoke with cvmmap://zed1, and remote execution on 192.168.2.155 using the deployed bundle lib directory for the GCC 15 runtime.

docs/caveats.md

@@ -16,30 +16,36 @@ Legacy flags such as `--shm-name`, `--zmq-endpoint`, `--input-mode`, and the dum
 
 ## Encoder Path
 
-### FFmpeg Is The Only Encoder Backend
+### Jetson Hardware Encode Uses The Jetson Multimedia API Backend
 
-The public backend surface is:
+The public backend surface is still:
 
 - `--encoder-backend auto`
 - `--encoder-backend ffmpeg`
 
-Both resolve to the FFmpeg encoder path. The removed GStreamer backend is no longer available.
+`--encoder-backend ffmpeg` keeps the existing FFmpeg encoder path.
+`--encoder-backend auto` may now select the in-repo Jetson Multimedia API backend on Jetson builds when hardware encoding is requested.
 
-### NVENC Is Optional
+The removed GStreamer backend is still unavailable.
 
-When `--encoder-device nvidia` is selected, FFmpeg must expose `h264_nvenc` and `hevc_nvenc`.
+### NVIDIA Device Mode On Jetson No Longer Depends On FFmpeg Hardware Encoders
 
-Useful local checks:
+On Jetson builds with `CVMMAP_STREAMER_ENABLE_JETSON_MM` enabled, `--encoder-device nvidia` uses the Jetson Multimedia API encoder directly rather than desktop `*_nvenc` or Jetson FFmpeg `*_v4l2m2m` encoder exposure.
+
+Selection behavior is now:
+
+- `--encoder-device software` -> FFmpeg software encode (`libx264`/`libx265`)
+- `--encoder-device nvidia` -> Jetson Multimedia API backend only; fail if unavailable
+- `--encoder-device auto` -> try Jetson Multimedia API first on Jetson builds, then fall back to FFmpeg software
+
+Useful local checks on Jetson:
 
 ```bash
-ffmpeg -hide_banner -encoders | rg 'nvenc|libx264|libx265'
+test -e /dev/v4l2-nvenc
+gst-inspect-1.0 nvv4l2h264enc nvv4l2h265enc
 ```
 
-If NVENC is unavailable, use:
-
-```bash
---encoder-device software
-```
+FFmpeg encoder enumeration is no longer the authoritative Jetson hardware-encode check for this repo.
 
 ### Low-Latency Defaults
 
@@ -47,7 +53,7 @@ The current low-latency defaults are:
 
 - `gop=30`
 - `b_frames=0`
-- NVENC preset/tune tuned for low latency
+- encoder-family-specific low-latency options where FFmpeg exposes them
 
 This favors immediacy over compression efficiency.
 

docs/compat_matrix.md

@@ -27,7 +27,7 @@ Notes:
 
 - RTMP is Enhanced RTMP only.
 - The custom RTMP packetizer and domestic mode are removed.
-- `encoder.backend` remains `auto|ffmpeg`; both resolve to FFmpeg.
+- `encoder.backend` remains `auto|ffmpeg`; `auto` may select Jetson Multimedia API on Jetson builds, while `ffmpeg` forces the FFmpeg encoder path.
 
 ## Optional Checks (Non-Blocking)
 
@@ -60,7 +60,7 @@ Current recording scope:
 
 | Setting | Value |
 |---------|-------|
-| Encoder backend | `auto` -> FFmpeg |
+| Encoder backend | `auto` -> Jetson MM on Jetson hardware requests, else FFmpeg |
 | RTMP transport | `libavformat` |
 | RTMP mode | Enhanced only |
 | Encoder device | `auto` |