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.
• 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

Automated validation

• GitHub Actions: all required checks pass.
• 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: 08e7e9cd22ad5dcd5465536caa6f0b200eb12486
• Adafruit ZeroDMA: 25e92a0811b364a068989c43788727c18803fc77