fix(stream): /critico review fixes for the sidecar cache

- ExtractSubtitlesVTTMulti: distrust output when ffmpeg is killed by signal
  (45-min timeout on a too-big remux) — a truncated WebVTT passed the len>0
  check and got cached as a silently-incomplete track until the media mtime
  changed. Skip all output on signal-kill; keep it on a clean non-zero exit.
- stream handlers: read the sidecar cache BEFORE the ffmpegPath guard so a
  pre-warmed sub/thumbnail still serves if ffmpeg was removed after the cache
  was filled.
- scan: log when the prewarm is skipped because ffmpeg is unavailable (matches
  the daemon; CLAUDE.md wants bootstrap to log on every branch).
- unexport sidecarDir/subtitleCachePath/thumbnailCachePath (no external callers).
- prewarm: surface a sample error in the summary so a systemic ffmpeg failure
  is distinguishable from one corrupt file.
- add unit tests: codec whitelist, cache paths, mtime freshness, atomic write,
  thumb-position dedup.

internal/library/prewarm_test.go

@@ -0,0 +1,36 @@
+package library
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/torrentclaw/unarr/internal/library/mediainfo"
+)
+
+func itemWithDuration(d float64) LibraryItem {
+	return LibraryItem{
+		FilePath:  "/m/x.mkv",
+		MediaInfo: &mediainfo.MediaInfo{Video: &mediainfo.VideoInfo{Duration: d}},
+	}
+}
+
+func TestThumbPositions(t *testing.T) {
+	// Known duration → fractions (0.1/0.3/0.5/0.7/0.9) rounded to whole seconds.
+	if got := thumbPositions(itemWithDuration(1000)); !reflect.DeepEqual(got, []float64{100, 300, 500, 700, 900}) {
+		t.Errorf("dur=1000 → %v, want [100 300 500 700 900]", got)
+	}
+
+	// Unknown duration (no video info) → fixed fallback offsets.
+	if got := thumbPositions(itemWithDuration(0)); !reflect.DeepEqual(got, []float64{30, 120, 300, 600, 1200}) {
+		t.Errorf("dur=0 → %v, want fallback", got)
+	}
+	if got := thumbPositions(LibraryItem{FilePath: "/m/x.mkv"}); !reflect.DeepEqual(got, []float64{30, 120, 300, 600, 1200}) {
+		t.Errorf("nil MediaInfo → %v, want fallback", got)
+	}
+
+	// Very short clip → multiple fractions round to the same second; deduped.
+	// dur=2: round(0.2,0.6,1.0,1.4,1.8) = 0,1,1,1,2 → [0 1 2].
+	if got := thumbPositions(itemWithDuration(2)); !reflect.DeepEqual(got, []float64{0, 1, 2}) {
+		t.Errorf("dur=2 → %v, want [0 1 2] (deduped)", got)
+	}
+}