refactor: frame converter and mp4 track improvements

- Refactor frame converter implementation
- Update mp4 track to use ICodex
- General refactoring and code improvements

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

plugin/hls/pkg/codec.go

@@ -0,0 +1,11 @@
+package hls
+
+import (
+	"m7s.live/v5/pkg"
+	mpegts "m7s.live/v5/pkg/format/ts"
+)
+
+type VideoCodecCtx struct {
+	pkg.IVideoCodecCtx
+	mpegts.MpegtsPESFrame
+}