feat(spi): abort active slave transactions

[floitsch]

Summary

• Add a private spi_slave_abort_transaction API for the transaction currently mounted in the SPI target peripheral.
• Disconnect CS before requesting abort, and let the ISR retire the descriptor only after the peripheral and DMA no longer reference it.
• Serialize abort with natural completion so a racing completion cannot abort the next queued transaction.
• Delay classic ESP32 completion callbacks until the shared DMA reset has finished, making buffer ownership explicit and safe.
• Leave queued follow-up transactions intact and reconnect CS only after the next descriptor is fully armed.

Testing

• Built and linked downstream Toit firmware for classic ESP32.
• Ran the downstream two-board SPI target matrix, including DMA and non-DMA idle abort, active abort while CS remains asserted, and two recovery transfers on the same target.
• Stress-ran the autonomous buffer-target matrix five consecutive times during development, then reran it after closing the completion/re-arm race; it includes active close and immediate peripheral/pin reuse.
• Verified the asynchronous-controller child matrix on the same classic ESP32 rig.
• git diff --check.

The current ESP-IDF master checkout has no equivalent SPI target abort API; this is intentionally kept in the private driver interface.