POC: SDL-backed audio path for ncplayer

[rebroad]

Summary

• turn on -Werror globally so warnings fail fast, and export loglevel so helper objects (like the SDL backend) can link against the shared visibility symbol
• add src/media/audio-output.[ch]: SDL2 is dlopened on demand, exposes a ring buffer + clock helpers, and is only compiled into ncplayer; CMake now wires in SWRESAMPLE consistently
• extend the FFmpeg glue with an audio packet queue, pending-packet drain, resampler helpers, and logging so the decoder can survive EAGAIN without dropping audio
• update ncplayer to spin a dedicated audio thread, feed the new helpers, throttle logging, and drop video frames when lagging so audio keeps pace

Status

Proof-of-concept. Audio logging still hardcodes /tmp/ncplayer_audio.log, SDL2 is a runtime dependency for ncplayer, and the commits are intentionally chunky for early review. I plan to follow up by gating logging behind env/flags and splitting out the pacing/audio pieces further if this general direction sounds good.

Testing

• cmake --build build -j$(nproc) on Ubuntu 24.04 (FFmpeg 7.0, SDL 2.28)
• ./build/ncplayer (invocation without args) just to confirm the binary launches with the new audio plumbing