Initial plugin implementation

Track initial implementation of the plugin described in the README design spec: an OBS obs_output_info module (Rust cdylib) that opens an SRT listener socket and streams OBS's own Program output to a caller (e.g. SRT Matrix pulling a destination's program-out).

No implementation started yet — this is the tracking issue for when that work begins. See the README for architecture (plugin interface, AV ingestion via libobs encoder hooks, mpeg2ts muxing, srt-tokio transport) and the proposed Cargo.toml dependencies.

Replaces the manual Custom FFmpeg Output workaround documented on srt-matrix's OBS Setup Guide (Direction 3).