scream v0-rc4

Add --framerate CLI argument

Replaces the hardcoded 30 fps with a configurable value exposed via
--framerate (default 30). Wired through all appsrc caps, the feed loop
sleep interval, and the RTSP launch strings for both output and window modes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>