ports/stm: queue DMA halves via bitmask + grow audioio buffer

Replace the scalar buffer_half_to_fill with a halves_to_fill bitmask so a
back-to-back half/full IRQ pair queues both fills even if the background
callback hasn't run yet. Grow the DMA circular buffer to 8192 samples
(4096-sample halves) so each half-fill window covers ~186 ms at 22050 Hz,
giving the background callback enough slack to absorb SDIO cluster reads,
NeoPixel updates, and other main-loop stalls without underrun.

Also expand the audioio manual test suite (stereo_playback, serial
runner, README/TESTING docs) to cover the new behaviour.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>