feat(hls): persistent fMP4 segment cache + integrity + stats (0.9.7)

Cache keyed by sha256(absPath|quality|audioIdx)[:8] with .complete marker;
LRU + size-budget eviction; per-key writer-lock; pinned during play;
startup orphan reap; integrity verify on HIT; subtitle-completeness gate;
hit/miss counters + daily log line. New [downloads.hls_cache] block in
config.toml (enabled/size_gb/dir, default 5GB).

Smoke test: 2nd play of same source+quality is 23-31× faster (HIT path
skips ffmpeg entirely).

internal/cmd/version.go

@@ -1,4 +1,4 @@
 package cmd
 
 // Version is the CLI version. Overridden by goreleaser ldflags at release time.
-var Version = "0.9.6"
+var Version = "0.9.7"