feat(playback): draw the player timeline as chapter or programme segments

Phase 3 of the "Hybrid" player controls redesign: an optional
`timelineSegments` input (PlayerTimelineSegment { startSeconds,
endSeconds, title }) on app-player-controls. Segments are clamped,
ordered, overlap-cut and gap-filled into a cover of the duration, drawn as
proportional flex items with their own accent fill, and named in the
hover label (`Chapter 2 · 12:40`). No host supplies segments yet, so the
bar still renders as one segment; EPG catch-up programmes and mpv
chapters are separate follow-ups.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

.changes/playback-timeline-segments-model.md

@@ -0,0 +1,8 @@
+---
+type: internal
+area: playback
+---
+
+The shared player timeline can draw chapter or programme segments with
+per-segment progress and a titled hover label; no player supplies segments
+yet, so the bar still renders as one segment.