Load Jetson multimedia libraries at runtime instead of linking them

[stephen-derosa]

Overview

The Jetson MMAPI backend hard-linked libnvv4l2, libnvbufsurface and libv4l2,
so an aarch64 binary built with Jetson support failed to load on non-Jetson ARM
systems — the loader errors out before JetsonVideoEncoderFactory::IsSupported()
ever runs. It also pinned the build host's L4T SONAMEs.

This applies the same Implib.so lazy-loading treatment CUDA and VAAPI already use:

• Checked-in stubs for libv4l2.so / libnvbufsurface.so, generated from L4T r36.4.7.
• jetson_runtime_loader.{h,cpp} — the stubs' dlopen callback (searches the L4T vendor
  dirs, tolerates SONAME drift) and a lk_jetson_runtime_libs_available() probe.
• IsCodecSupported() and DmaBufVideoFrameBuffer::ToI420() check that probe before
  touching a lazily-bound symbol, since a stub aborts if its library is missing.
• LK_JETSON_MMAPI_DIR overrides the hardcoded /usr/src/jetson_multimedia_api.

Result: one portable aarch64 binary that uses the Jetson encoder on L4T and falls back
elsewhere.

Breaking changes

None. The backend already required JetPack 5 / L4T R34+ — it calls NvBufSurfaceFromFd
unconditionally, and libnvbufsurface first ships in R34.1. The removed nvbuf_utils
link was vestigial: no symbol from it was ever referenced, and JetPack 6 no longer ships
that library at all.

Testing

In a linux/arm64 container with no Jetson libraries, against the real L4T r36.4.7
sources and libraries: the vendored MMAPI objects link with -Wl,--no-undefined and
DT_NEEDED lists only libstdc++/libgcc_s/libc; the probe reports unavailable and exits
cleanly without the libraries, and resolves trampolined calls correctly with them.

Target HW Testing

tested on HW, works as expected
./target/release/publisher --camera-index 0 --format mjpeg --width 2560 --height 720 --fps 30 --codec h264
Opening in BLOCKING MODE
Jetson MMAPI encoder is supported.
Opening in BLOCKING MODE
Jetson MMAPI AV1 encoder available.
Opening in BLOCKING MODE
Jetson MMAPI encoder is supported.
Using Jetson MMAPI encoder for H264
Opening in BLOCKING MODE
NvMMLiteOpen : Block : BlockType = 4
===== NvVideo: NVENC =====
NvMMLiteBlockCreate : Block : BlockType = 4
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H264: Profile = 66 Level = 31
NVMEDIA: Need to set EMC bandwidth : 846000
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NvVideo: bBlitMode is set to TRUE
LK_DUMP_H264 not set; skipping H264 dump (MMAPI).
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Opening in BLOCKING MODE
NvMMLiteOpen : Block : BlockType = 4
===== NvVideo: NVENC =====
NvMMLiteBlockCreate : Block : BlockType = 4
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842091865
H264: Profile = 66 Level = 31
NVMEDIA: Need to set EMC bandwidth : 846000
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[github-actions]

Changeset ✓

This PR includes a changeset covering all affected packages:

Package	Bump
libwebrtc	patch
livekit	patch
livekit-ffi	patch
webrtc-sys	patch

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.