feat(hls): pre-segmentación delantada — 2 s segments + async session start (0.9.10)

First-frame latency drops by another 1-2 s on cold-cache plays:

1. HLS segment duration halved from 4 s to 2 s. seg-0 lands in ~half
   the wait time — the player paints the first frame as soon as it
   arrives. Software encodes on 4K go from ~3 s wait to ~1.5 s; HW
   encoders shave ~0.5 s. Trade-off: 2× segment count per source
   (~3600 segments for a 2 h movie instead of ~1800), but each is
   half the size on disk. Within HLS spec — Apple recommends 6 s, but
   2 s is valid; LL-HLS uses 1-2 s.

2. Cache from 0.9.9 self-heals: cached entries used 4 s segments;
   VerifyComplete now expects a different highest segment index and
   invalidates them, triggering a re-encode on next play. No manual
   cleanup needed.

3. OnStreamSession daemon callback now runs StartHLSSession in a
   goroutine. Sync HTTP responses return immediately (~50 ms instead
   of waiting for the ~0.3-1 s ffprobe). Other pending actions in
   the same sync cycle (new tasks, deletes) no longer wait for the
   transcoder warmup. Browser HEAD probes already have a 30 s retry
   budget that covers the brief gap between playerSessionRegistry.add
   and streamSrv.HLS().Register.

Helpers added (engine.segmentDurationFor / segmentStartSec /
segmentCountForDuration) so a future short-first-segment variant or
non-uniform layout can slot in without touching every call site.

Internal: -hls_init_time was investigated but discarded — ffmpeg's
implementation treats it as a min duration, not a target, so it
couldn't deliver a uniformly 2 s first segment on top of a 4 s
steady state. Uniform 2 s is simpler and gets the same first-frame
win.

internal/engine/hls_test.go

@@ -115,10 +115,11 @@ func TestRenderVideoPlaylist(t *testing.T) {
 }
 
 func TestRenderVideoPlaylistShortFinalSegment(t *testing.T) {
-	// 9.5s total, 4s segments → 3 segs of 4/4/1.5
-	out := renderVideoPlaylist(9.5, 3)
+	// 9.5s total, 2s segments → 5 segs of 2/2/2/2/1.5
+	segCount := segmentCountForDuration(9.5)
+	out := renderVideoPlaylist(9.5, segCount)
 	if !strings.Contains(out, "#EXTINF:1.500,") {
-		t.Errorf("expected final segment 1.5s in playlist, got:\n%s", out)
+		t.Errorf("expected final segment 1.5s in playlist (segCount=%d), got:\n%s", segCount, out)
 	}
 }