SERCOM async/DMA with SAME5x support

[crabel99]

Summary

• Extend the existing SERCOM asynchronous/DMA work across SAMD21 and SAME5x.
• Retain asynchronous Wire, SPI, and UART transaction support.
• Make SPI.end() abort active transfers and clear queued source pointers before callers release their buffers. Preserve the incoming interrupt-mask state.
• Add SAME53/SAME54 core, interrupt, timing, USB, and peripheral-family guards.
• Add multimaster Wire role transitions, slave DMA, transaction chaining, and bounded error recovery.
• Pin the reviewed TinyUSB and ZeroDMA dependency revisions.

This replaces #385. The original PR remains available for its discussion and history, but its sercom-async-dma head diverged before the SAME5x integration work. This replacement allows CI and review to run against the branch containing the SAME5x integration without rewriting the old public branch.

Validation

Current SPI fix validation

• GitHub CI: all 12 jobs pass on the updated PR head.

• Full local 12-job CI matrix: 287 successful example compiles, zero failures, and 433 existing skips at 76492a7993f63a2c4687fcdeb5a57853a2272598.

• Native SPI buffer-lifetime reproduction: all eight cases pass.

Earlier validation

• GitHub Actions passed all required checks on the preceding PR head, 0b814e16.
• Native and contract tests: 32 of 32 pass against the pinned dependency revisions.
• Metro M0 example matrix: 24 passed, 0 failed, 37 skipped.
• Metro M4 example matrix: 20 passed, 0 failed, 41 skipped.
• SAME54 Xplained production configuration builds successfully.
• SparkFun Qwiic Micro SAMD21 production configuration builds successfully.

Hardware validation

• MCUs: SAMD21G18A and SAME54P20A.
• Configuration: two-node I2C bus with dynamic master/slave role reversal.
• Covered asynchronous writes and reads, combined write/read transactions, repeated START, DMA and byte-driven transfers, queued transfers, and error-recovery paths.
• The complete 17-case hardware matrix passed with the pinned dependency revisions.

Dependencies

• Adafruit TinyUSB Arduino: 057f6734f589680072de3a467b3b4051e57f3a9e
• Adafruit ZeroDMA: 25e92a0811b364a068989c43788727c18803fc77

SPI buffer lifetime regression

SPI.end() previously left the DMA engine and queued transaction pointers active after returning. The change to SERCOM::resetSPI() cancels transfer activity and clears those references before releasing caller-owned memory.

The native reproduction failed against the previous implementation and passes against this commit. A SAME54 J-Link probe independently reproduced the stale DMA ownership and verified cancellation after the fix. This also supports NeoPixel SERCOM output, which retains each encoded source buffer until SPI completion.