fedac/native/audio.c: bump piano sample amp 1.8 → 3.0
Piano voice was still noticeably quieter than sine/square at the
same notepat `volume` — the Salamander samples sit several dB below
the ±1 oscillators and 1.8× wasn't enough headroom recovery. Match
harp's boosted philosophy (LPF + stretch attenuates the loop, so it
gets 2.5×) and let soft_clip catch any peak excess.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>