perf(epg): render only the timeline programmes near the visible range (J3)

Selecting a live channel rendered every programme block of its schedule
(about 240 for the Xtream mock) while the stream was starting: about 6,000
of J3's 6,199 renderer.domMutationsToPlaying. The ribbon now renders the
blocks, ticks and day dividers within half a viewport of the visible range;
the track keeps the full schedule's width, so positions, the scrollbar and
scroll-to-now are unchanged.

A resize reported before the scroll-to-now must not re-centre the window
(it once jumped to the schedule's start and back); resizes only widen it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.changes/epg-timeline-render-near-view.md

@@ -0,0 +1,8 @@
+---
+type: perf
+area: epg
+---
+
+Live channels start playing with less work competing for the first frame: the
+programme guide below the player now draws only the programmes around the part
+you are looking at, and fills in the rest as you scroll.