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.

Optional Wire CRC integration

Wire now supports opt-in CRC-32/ISO-HDLC for each data-bearing I2C phase. The four-byte little-endian trailer is separate from all 255 payload bytes. Address-only probes carry no trailer. CRC remains disabled by default; SPI and UART are unchanged by this integration. Framework retains DMA selection, CRC engine ownership and physical completion. Interrupt-driven phases feed CRCDATAIN rather than silently omitting protection. Failed or truncated receives deliver zero bytes.

The shared DmacCrc dependency is merged from #398. Two preceding receive-retirement guards are a separate fix commit. A native reproduction failed before that correction and passed afterward; paired D21/E54 repeated-START tests also passed at the original full-buffer boundary.

Additional validation includes all 43 Framework native cases; 106 paired corruption/truncation cases with valid 255-byte recovery in both bus directions; three full-capacity repeated-START phases; address-NACK recovery; and 100 iterations of the older slave-TX stall reproduction at 100 kHz. Device acceptance verified real register rejection, signed firmware validation, unchanged inactive flash after every rejected page, and unchanged flash/offset after valid-page replay. Hardware tests used the integrated bundle's matching CRC source.

All 12 local Arduino CI configurations pass on exact branch commit 431bd1931c53a27a0d7fca257a9691d4395874cd, including TinyUSB builds using that branch's own pinned library. Earlier interrupted/overridden-library runs remain recorded separately.