ambient: polyphonic songs, fix cascaded gain and resume bugs

Rewrite all 19 theme melodies into proper ~10-16 second looping songs
with bass drones, chord pads, and scalar melody lines using the new
'step' field for polyphonic overlap on a monophonic step sequencer.

Fix four bugs that caused total silence:
1. masterGain was faded to preset.gain while each note was ALSO scaled
   by preset.gain*0.6, squaring volume to ~-68 dB.
2. fadeMasterTo ramped the gate for 0.5-1.5s, swallowing first notes.
3. startDrones set per-osc gain to 1.0/numDrones, clipping and burying
   the melody.
4. AudioContext resume was not awaited; notes fired into a suspended
   context and were lost.

Shared.js:
- startEngine() waits for ctx.resume() before scheduling anything.
- masterGain acts as a binary gate (fades to 1.0 over 50ms).
- playMelodyNote uses twin detuned oscillators for richer timbre.
- startDrones scales by preset.gain so drones match melody level.

Theme_sounds.go:
- Add 'step' field to melodyNote struct.
- Add chord-building helpers (major/minor triads).
- Add buildSong() / buildMeasure() for structured 4-bar loops.
- All 19 themes: audible bass (130-220Hz), chord pads (260-520Hz),
  melodies (330-1046Hz), real chord progressions, proper BPM.

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