Fix synthio_biquad_filter_reset() clearing only half the filter state

[bdbarnett]

Fixes #11268.

shared-module/synthio/Biquad.h defines the filter state as four
int32_t:

typedef struct {
    int32_t x[2], y[2];
} biquad_filter_state;

but shared-module/synthio/Biquad.c only clears the first two:

void synthio_biquad_filter_reset(biquad_filter_state *st) {
    memset(&st->x, 0, 4 * sizeof(int16_t));
}

The struct is 16 bytes; 4 * sizeof(int16_t) is 8. x[0] and x[1] get
cleared, but y[0] and y[1] -- the feedback history -- do not.
biquad_filter_sample() reads all four members as int32_t, so the
int16_t sizing looks like a leftover from an earlier, narrower state
layout.

A biquad's y history is what the recursion runs on, so what survives a
"reset" is not a cosmetic detail: feeding a reset filter pure silence
produces a decaying tail of whatever it played before, and for a low-pass
with poles near the unit circle that tail starts close to full scale.
Both callers -- audiofilters_filter_reset_buffer() (called when an
AudioOut starts playback) and synthio.Note's filter (re)initialisation
-- want the full reset.

Repro

An 800 Hz low-pass fed a loud 200 Hz tone for four blocks, then
reset_buffer(), then silence. Peak of the first "silent" block, unix
coverage build of main:

	peak out (int16)
before	28082 (-1.3 dBFS)
after	0

Includes a regression test
(tests/circuitpython/audiofilter_filter_reset_buffer.py) with the same
scenario; it fails on current main and passes with the fix.

[tannewt]

Thank you!