refactor(osc): support held-note parameter updates without phase reset

Add Oscillator::setParams() which updates detune, pan, and gain for
already-initialised sub-voices without resetting their phases. This
preserves phase continuity when unison or spread is LFO-modulated at
control rate. noteOn() now just seeds phases and delegates to
setParams().

Precompute panL/panR in SubVoice instead of calling cos/sin per
sample. Add paramsValid and cachedParams to skip redundant work.

Replace SynthVoice's lastUnisonA/B tracking with a single
oscillatorsNeedNoteOn flag: noteOn sets it, render clears it after
the first block. Add SubLevel and NoiseLevel to the per-voice
modulation targets so they can be modulated like other parameters.

Source/Oscillator.h

@@ -41,8 +41,9 @@ public:
     void prepare (double sampleRate)          { sr = sampleRate; reset(); }
     void reset();
 
-    // (Re)configure unison sub-voices: phase offsets, detune, pan and gain.
+    // Initialise a fresh note's unison sub-voices: phase offsets, detune, pan and gain.
     void noteOn (double freqHz, const OscParams& p, juce::uint32 seed);
+    void setParams (const OscParams& p) noexcept;
 
     // Accumulate this oscillator's contribution into outL/outR.
     void processAdd (const Wavetable& wt, const OscParams& p, double freqHz,
@@ -55,16 +56,20 @@ private:
     {
         double phase       = 0.0;
         double detuneRatio = 1.0;
-        float  pan         = 0.0f;
+        float  panL        = 0.70710678f;
+        float  panR        = 0.70710678f;
         float  level       = 1.0f;
     };
 
     std::array<SubVoice, kMaxUnison> voices;
     int    activeUnison = 1;
+    int    initializedUnison = 0;
+    bool   paramsValid = false;
+    OscParams cachedParams;
     double sr           = 44100.0;
     juce::Random rng;
 
-    static constexpr double kTwoPi = 6.28318530717958647692;
+    static constexpr double kTwoPi = juce::MathConstants<double>::twoPi;
 
     float warpPhase (float phase, const OscParams& p) const noexcept;
     float warpSample (float sample, const OscParams& p) const noexcept;