Document comma chestnut hardware and troubleshooting

[nelsonjchen]

Adds a comma chestnut section with official comma four setup guidance and three sourced troubleshooting cases: GPU detection and slow USB, power interruption during engine start, and GPS interference in community C3X configurations. Includes the Chestnut Discord channel, a cross-reference from the existing GPS case, and the requested editor's note inviting a chestnut owner to help test and maintain the section.

Distinguishes the stable release from newer development diagnostics, preserves the reported limits of community fixes, and describes restarting after restoring power as recovery. The engine-crank issue remains open and the retry proposal is closed without merging, as checked on 2026-10-03.

Validation completed:

• Reviewed comma's blog and shop/setup guide, exact Discord reports, pinned source code, and the linked issue/PR outcomes for citations and compatibility wording.
• Regenerated the TOC with pnpm dlx doctoc README.md --github.
• Passed git diff --check.
• Verified all new anchors and both GPS cross-reference targets resolve uniquely.
• Confirmed the prohibited ODB typo is absent.
• Confirmed the PR diff changes only README.md for Chestnut documentation; the earlier flashing/fuse branch is separate.
• GitHub lint checks passed for both the branch push and pull request.

Documentation review only; no hardware testing was performed.

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