#4305 @swamp/video/brand: cap each Chromium stderr line kept for errors

Follow-up from verify-reviews on swamp-club #4124 (commit 7fe231c43), low severity. StderrTail in video/brand/extensions/models/_lib/scene.ts caps only the trailing partial line at STDERR_LINE_MAX (4096). Completed lines are pushed whole, so a Chromium that writes a few 64 KB newline-terminated lines puts up to about 15 x 64 KB into every error built from tail(). Memory stays bounded (15 lines). Also push only parts.slice(-STDERR_TAIL_LINES) rather than spreading every split line into push(). Fix: cap each line before pushing; add a test with long newline-terminated lines.