fix: implement nRF52 RTTTL player using direct NRF_PWM2 control for working volume

tone() dynamically computes seq_refresh so the DMA repeats 50% duty for
~13-30ms per note before re-reading its buffer — volume cannot take effect
until that delay passes, producing a staircase artifact.

Replace tone()/NonBlockingRtttl with a custom RTTTL parser that drives
NRF_PWM2 directly using REFRESH=0. DMA now re-reads _duty_buf every
PWM period so:
- Duty is set before SEQSTART fires → no initial full-volume burst
- applyVolume() writes to _duty_buf and takes effect within one period (<2.3ms at A4)
- setVolume() during playback adjusts loudness immediately and smoothly

Non-nRF52 platforms continue to use NonBlockingRtttl + analogWrite.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/helpers/ui/buzzer.h

@@ -1,43 +1,58 @@
 #pragma once
 
 #include <Arduino.h>
-#include <NonBlockingRtttl.h>
 
-/* class abstracts underlying RTTTL library 
-
-    Just a simple imlementation to start.  At the moment use same
-    melody for message and discovery
-    Suggest enum type for different sounds
-    - on message
-    - on discovery
-
-    TODO
-    - make message ring tone configurable
-
-*/
+#if !defined(NRF52_PLATFORM)
+  #include <NonBlockingRtttl.h>
+#endif
 
 class genericBuzzer
 {
     public:
-        void begin();  // set up buzzer port
+        void begin();
         void play(const char *melody);
-        void playForced(const char *melody);  // play regardless of quiet state
-        void loop();  // loop driven-nonblocking
-        void startup();  // play startup sound
-        void shutdown();  // play shutdown sound
-        bool isPlaying();  // returns true if a sound is still playing else false
-        void quiet(bool buzzer_state);  // enables or disables the buzzer
-        bool isQuiet();  // get buzzer state on/off
-        void setVolume(uint8_t level);  // 0=min..4=max
+        void playForced(const char *melody);
+        void loop();
+        void startup();
+        void shutdown();
+        bool isPlaying();
+        void quiet(bool buzzer_state);
+        bool isQuiet();
+        void setVolume(uint8_t level);
         uint8_t getVolume() const { return _volume_level; }
-        void stop();  // stop any playing melody
+        void stop();
 
     private:
         uint8_t _volume_level = 4;
         void applyVolume();
-        // gemini's picks:
-        const char *startup_song = "Startup:d=4,o=5,b=160:16c6,16e6,8g6";
+        const char *startup_song  = "Startup:d=4,o=5,b=160:16c6,16e6,8g6";
         const char *shutdown_song = "Shutdown:d=4,o=5,b=100:8g5,16e5,16c5";
-
         bool _is_quiet = true;
+
+#if defined(NRF52_PLATFORM)
+        // Own RTTTL player — bypasses tone() to allow volume control from note start.
+        // tone() pre-computes seq_refresh so the DMA repeats 50% duty for ~30ms before
+        // re-reading its buffer; we cannot beat that timing.  By owning NRF_PWM2 directly
+        // and setting REFRESH=0, DMA re-reads _duty_buf every period so duty takes effect
+        // immediately at SEQSTART.
+        volatile uint16_t _duty_buf = 0;   // DMA source — must stay in RAM
+        const char*   _rtttl_pos   = nullptr;
+        bool          _rtttl_done  = true;
+        bool          _pwm_on      = false;
+        unsigned long _note_end_ms = 0;
+        uint8_t       _def_dur     = 4;
+        uint8_t       _def_oct     = 5;
+        uint16_t      _def_bpm     = 120;
+
+        void    _nrfBegin(const char* melody);
+        void    _nrfAdvance();
+        void    _nrfStartPwm(uint16_t freq);
+        void    _nrfStopPwm();
+        uint8_t _dutyPct() const;
+        static uint16_t _noteFreq(char letter, bool sharp, uint8_t octave);
+        static bool     _parseNext(const char*& pos, uint8_t def_dur, uint8_t def_oct,
+                                   uint16_t bpm, uint16_t& freq_hz, uint32_t& dur_ms);
+        static void     _parseHeader(const char* melody, uint8_t& def_dur, uint8_t& def_oct,
+                                     uint16_t& bpm, const char*& notes_start);
+#endif
 };