feat(hls): faster first-start — probe cache + tighter encoder presets (0.9.9)

Reduces first-segment latency on cache MISS so the player doesn't sit on
"preparando sesión". Three independent levers:

1. ProbeFile memoised by (path, mtime, size) for 30 min — second play of
   the same source skips ffprobe (1-3 s on 50+ GB MKVs).
2. HLS encoder presets biased for latency over quality:
   - libx264 default veryfast → superfast (~15-20% faster, marginal
     quality loss at 5-25 Mbps target bitrates).
   - NVENC: -preset p4 -tune hq → -preset p3 -tune ll. First-segment
     ~0.8 s on RTX-class GPUs (was ~1.5 s).
   - QSV: -preset medium → -preset veryfast (keeps look_ahead=0).
   - VideoToolbox: adds -realtime 1 (was unset). Bitrate args still
     drive rate control; -q:v dropped to avoid the silent conflict
     where ffmpeg ignored it under -b:v.
3. Per-session log surfaces encoder + accel + preset so "first-start
   was slow" complaints can be triaged from the journal alone.

Diagnostic helpers (DetectHWAccelDiagnostic + HWAccelDiagnostic) added
for future wiring into daemon startup / agent register; users today can
already inspect via `unarr probe-hwaccel`.

Web: AgentsTab profile page now shows the agent's chosen encoder
(amber if software libx264, green if HW) plus the transcode-resolution
cap. Hidden for pre-0.9.9 agents that haven't reported hwAccel.

internal/engine/probe_cache_test.go

@@ -0,0 +1,154 @@
+package engine
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+func TestProbeCache_LookupMissNonexistent(t *testing.T) {
+	ResetProbeCache()
+	t.Cleanup(ResetProbeCache)
+
+	if _, ok := lookupProbeCache("/path/that/does/not/exist"); ok {
+		t.Fatal("expected MISS for non-existent path")
+	}
+}
+
+func TestProbeCache_StoreThenLookupHit(t *testing.T) {
+	ResetProbeCache()
+	t.Cleanup(ResetProbeCache)
+
+	dir := t.TempDir()
+	path := filepath.Join(dir, "movie.mkv")
+	if err := os.WriteFile(path, []byte("fake content"), 0o644); err != nil {
+		t.Fatalf("write tmp file: %v", err)
+	}
+
+	probe := &StreamProbe{VideoCodec: "h264", Width: 1920, Height: 1080, DurationSec: 5400}
+	storeProbeCache(path, probe)
+
+	got, ok := lookupProbeCache(path)
+	if !ok {
+		t.Fatal("expected HIT after store")
+	}
+	if got != probe {
+		t.Fatalf("expected pointer-identical probe; got different")
+	}
+}
+
+func TestProbeCache_MtimeChangeInvalidates(t *testing.T) {
+	ResetProbeCache()
+	t.Cleanup(ResetProbeCache)
+
+	dir := t.TempDir()
+	path := filepath.Join(dir, "movie.mkv")
+	if err := os.WriteFile(path, []byte("original"), 0o644); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+
+	probe := &StreamProbe{VideoCodec: "h264", DurationSec: 100}
+	storeProbeCache(path, probe)
+
+	// Force mtime change. WriteFile doesn't guarantee a different mtime if
+	// the filesystem timestamp resolution is coarse, so set it explicitly
+	// to a value 1 hour in the future.
+	future := time.Now().Add(1 * time.Hour)
+	if err := os.Chtimes(path, future, future); err != nil {
+		t.Fatalf("chtimes: %v", err)
+	}
+
+	if _, ok := lookupProbeCache(path); ok {
+		t.Fatal("expected MISS after mtime change")
+	}
+}
+
+func TestProbeCache_SizeChangeInvalidates(t *testing.T) {
+	ResetProbeCache()
+	t.Cleanup(ResetProbeCache)
+
+	dir := t.TempDir()
+	path := filepath.Join(dir, "movie.mkv")
+	if err := os.WriteFile(path, []byte("aaaaa"), 0o644); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+
+	probe := &StreamProbe{VideoCodec: "h264", DurationSec: 100}
+	storeProbeCache(path, probe)
+
+	// Truncate file to a different size + reset mtime to the original (to
+	// isolate the size-check path). Stat picks up new size immediately.
+	if err := os.WriteFile(path, []byte("a"), 0o644); err != nil {
+		t.Fatalf("rewrite: %v", err)
+	}
+
+	if _, ok := lookupProbeCache(path); ok {
+		t.Fatal("expected MISS after size change")
+	}
+}
+
+func TestProbeCache_ExpiryDropsEntry(t *testing.T) {
+	ResetProbeCache()
+	t.Cleanup(ResetProbeCache)
+
+	dir := t.TempDir()
+	path := filepath.Join(dir, "movie.mkv")
+	if err := os.WriteFile(path, []byte("content"), 0o644); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+
+	// Stash an entry whose expires is already in the past — simulates TTL
+	// having elapsed without sleeping for 30 min.
+	fi, err := os.Stat(path)
+	if err != nil {
+		t.Fatalf("stat: %v", err)
+	}
+	key := probeCacheKey{path: path, mtime: fi.ModTime().UnixNano(), size: fi.Size()}
+	probeCacheMu.Lock()
+	probeCache[key] = probeCacheEntry{
+		probe:   &StreamProbe{VideoCodec: "h264"},
+		expires: time.Now().Add(-1 * time.Minute),
+	}
+	probeCacheMu.Unlock()
+
+	if _, ok := lookupProbeCache(path); ok {
+		t.Fatal("expected MISS for expired entry")
+	}
+	// Side-effect: lookup should have evicted the stale entry.
+	if ProbeCacheSize() != 0 {
+		t.Fatalf("expected cache size 0 after expiry eviction; got %d", ProbeCacheSize())
+	}
+}
+
+func TestProbeCache_ResetClears(t *testing.T) {
+	ResetProbeCache()
+
+	dir := t.TempDir()
+	path := filepath.Join(dir, "movie.mkv")
+	if err := os.WriteFile(path, []byte("x"), 0o644); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+
+	storeProbeCache(path, &StreamProbe{VideoCodec: "h264"})
+	if ProbeCacheSize() != 1 {
+		t.Fatalf("expected size 1 after store; got %d", ProbeCacheSize())
+	}
+
+	ResetProbeCache()
+	if ProbeCacheSize() != 0 {
+		t.Fatalf("expected size 0 after reset; got %d", ProbeCacheSize())
+	}
+}
+
+func TestProbeCache_StoreNonexistentNoOp(t *testing.T) {
+	ResetProbeCache()
+	t.Cleanup(ResetProbeCache)
+
+	// Store on a non-existent path should silently do nothing (stat fails),
+	// not panic, and not poison the cache with a zero key.
+	storeProbeCache("/nope/never/exists.mkv", &StreamProbe{VideoCodec: "h264"})
+	if ProbeCacheSize() != 0 {
+		t.Fatalf("expected 0 entries; got %d", ProbeCacheSize())
+	}
+}