#4446 @swamp/video: small follow-ups from joining takes

Non-blocking review findings from swamp-club#4330, none of which produces a wrong video. (1) The measuring loop in render.ts repeats what takesOf does. (2) A joined render's recording field is a stem-plus-.mp4 path that is never written; the real sources are in takes, so document this on the schema field or store something real. (3) With takes containing x.polished.mp4 and output x, the render's output is also an input: ffmpeg should refuse, but the error is confusing and comes after the measuring passes, so refuse it in renderTargets. (4) A media instance with no bed set is only caught at render, after transcription and screen analysis, though the check-bed step describes failing before the slow steps: read the media settings early, or narrow the description.