An on-demand stream emits short (2s) fast-start segments before it ramps up to
seg_time. getPlaylistSegments picked intval(client_prebuffer / seg_time)
segments -- with the default 10s prebuffer / 10s nominal seg_time that is 1
segment, i.e. only ~2s of actual buffer at cold start, which players surfaced as
"caches 2 seconds". Persistent streams (segments already ~10s) were unaffected:
1 segment = 10s.
Select the newest segments whose #EXTINF durations sum to at least the requested
prebuffer seconds instead, so client_prebuffer means real seconds regardless of
segment length. Fast-start (hls_init_time 2) is kept. Falls back to the old
nominal count when the playlist has no #EXTINF lines; the -1 (all) and 0 (index)
modes are unchanged. Split into a pure selectSegments()/parseSegmentDurations()
so the selection is unit tested.
Verified: phpstan level 5 green, phpunit 405/405, make gates green.
