fix(stereo): apply mode changes at chunk boundaries

Stop reconfiguring the stereo encoder from UpdateLive, stage canonical stereoMode values as desired state, and apply SetMode only from the DSP thread at the start of GenerateFrame. Add tests covering deferred mode application and mode-name canonicalization.

internal/offline/generator.go

@@ -85,15 +85,16 @@ type Generator struct {
 	cfg cfgpkg.Config
 
 	// Persistent DSP state across GenerateFrame calls
-	source        *PreEmphasizedSource
-	stereoEncoder stereo.StereoEncoder
-	rdsEnc        *rds.Encoder
-	rds2Enc       *rds.RDS2Encoder
-	combiner      mpx.DefaultCombiner
-	fmMod         *dsp.FMModulator
-	sampleRate    float64
-	initialized   bool
-	frameSeq      uint64
+	source            *PreEmphasizedSource
+	stereoEncoder     stereo.StereoEncoder
+	appliedStereoMode string // canonical mode currently applied to stereoEncoder; DSP goroutine only
+	rdsEnc            *rds.Encoder
+	rds2Enc           *rds.RDS2Encoder
+	combiner          mpx.DefaultCombiner
+	fmMod             *dsp.FMModulator
+	sampleRate        float64
+	initialized       bool
+	frameSeq          uint64
 
 	// Broadcast-standard clip-filter-clip chain (per channel L/R):
 	//
@@ -178,13 +179,10 @@ func (g *Generator) SetExternalSource(src frameSource) error {
 	return nil
 }
 
-// UpdateLive hot-swaps DSP parameters. Thread-safe — called from control API,
-// applied at the next chunk boundary by the DSP goroutine.
+// UpdateLive hot-swaps DSP parameters. Thread-safe — called from control API.
+// The DSP goroutine applies mode changes at the next chunk boundary.
 func (g *Generator) UpdateLive(p LiveParams) {
-	// Detect stereo mode change: requires reconfiguring the encoder's Hilbert filter.
-	if old := g.liveParams.Load(); old != nil && old.StereoMode != p.StereoMode {
-		g.stereoEncoder.SetMode(stereo.ParseMode(p.StereoMode), g.sampleRate)
-	}
+	p.StereoMode = canonicalStereoMode(p.StereoMode)
 	g.liveParams.Store(&p)
 }
 
@@ -197,6 +195,10 @@ func (g *Generator) CurrentLiveParams() LiveParams {
 	return LiveParams{OutputDrive: 1.0, LimiterCeiling: 1.0, MpxGain: 1.0}
 }
 
+func canonicalStereoMode(mode string) string {
+	return stereo.ParseMode(mode).String()
+}
+
 // RDSEncoder returns the live RDS encoder, or nil if RDS is disabled.
 // Used by the Engine to forward text updates.
 func (g *Generator) RDSEncoder() *rds.Encoder {
@@ -223,7 +225,8 @@ func (g *Generator) init() {
 	rawSource, _ := g.sourceFor(g.sampleRate)
 	g.source = NewPreEmphasizedSource(rawSource, g.cfg.FM.PreEmphasisTauUS, g.sampleRate, g.cfg.Audio.Gain)
 	g.stereoEncoder = stereo.NewStereoEncoder(g.sampleRate)
-	g.stereoEncoder.SetMode(stereo.ParseMode(g.cfg.FM.StereoMode), g.sampleRate)
+	g.appliedStereoMode = canonicalStereoMode(g.cfg.FM.StereoMode)
+	g.stereoEncoder.SetMode(stereo.ParseMode(g.appliedStereoMode), g.sampleRate)
 	g.combiner = mpx.DefaultCombiner{
 		MonoGain: 1.0, StereoGain: 1.0,
 		PilotGain: g.cfg.FM.PilotLevel, RDSGain: g.cfg.FM.RDSInjection,
@@ -335,7 +338,7 @@ func (g *Generator) init() {
 	g.liveParams.Store(&LiveParams{
 		OutputDrive:             g.cfg.FM.OutputDrive,
 		StereoEnabled:           g.cfg.FM.StereoEnabled,
-		StereoMode:              g.cfg.FM.StereoMode,
+		StereoMode:              g.appliedStereoMode,
 		PilotLevel:              g.cfg.FM.PilotLevel,
 		RDSInjection:            g.cfg.FM.RDSInjection,
 		RDSEnabled:              g.cfg.RDS.Enabled,
@@ -418,6 +421,11 @@ func (g *Generator) GenerateFrame(duration time.Duration) *output.CompositeFrame
 		lp = &LiveParams{OutputDrive: 1.0, LimiterCeiling: 1.0, MpxGain: 1.0}
 	}
 
+	if mode := canonicalStereoMode(lp.StereoMode); mode != g.appliedStereoMode {
+		g.stereoEncoder.SetMode(stereo.ParseMode(mode), g.sampleRate)
+		g.appliedStereoMode = mode
+	}
+
 	// Apply live tone and gain updates each chunk. GenerateFrame runs on a
 	// single goroutine so these field writes are safe without additional locking.
 	if g.toneSource != nil {