ports/stm: harden audioio review fixes + bump DMA to VERY_HIGH

Apply code-review fixes to the F405/F407 audioio implementation:

- Fix infinite loop on partial-frame source data in load_dma_buffer_half
  (was spinning when convert returned 0 with leftover bytes).
- Use canonical audiosample_get_* accessors for sample format.
- Validate sample_rate via mp_arg_validate_int_max (1 MHz ceiling).
- Replace m_malloc with m_malloc_without_collect to avoid GC during
  DAC configure.
- Raise on HAL_TIM_Base_Init / HAL_TIMEx_MasterConfigSynchronization /
  HAL_DMA_Init failure rather than silently continuing.
- Clear left/right pin refs and playing flag in audioout_reset so the
  next construct starts from a clean state.
- Gate paused on playing in get_paused, matching espressif convention.
- Claim pins first before any other allocation so the error path needs
  no rollback.
- Bump DMA priority HIGH -> VERY_HIGH on both streams (sweep analysis
  shows this is safe and gives more refill headroom).
- Make CIRCUITPY_AUDIOIO opt-out via ?= so boards reusing TIM6 / PA04
  can disable it.