V2 setCustomEQ wire format is pcap-verified: request payload is
[dir, slot, pad=0] + N × [freq_hi, freq_lo, filter, gain_hi, gain_lo].
Gain is offset-binary against the [gain_min, gain_max] / gain_steps
grid from getEQInfos. Freq and filter type aren't user-editable today;
they're sourced from the build-time cache.
Changes:
- rw_class.write: injects [dir, slot=active, pad=0] before band bytes
and re-queries the active slot each call (matches the read path).
- validator.prepare_write: encodes the band payload from the dict the
UI hands us, converting int dB → raw u16 against the gain grid.
- Setting.write: overrides RangeFieldSetting.write to treat empty-bytes
replies as success (HID++ set ops ACK with b""), only None as failure.
- Drop the read-only override and stale docstring.
be047fd949