ci(lychee): exclude the VS Code Marketplace, it returns 503 to automated requests

[sbryngelson]

Description

The Lychee link check fails on Build & Verify with a single error:

### Errors in build/install/docs/mfc/documentation/getting-started.html

* [503] <https://marketplace.visualstudio.com/items?itemName=ms-vscode-remote.remote-wsl> (at 218:86)
      | Rejected status code: 503 Service Unavailable

That is the "Remote - WSL" extension link at docs/documentation/getting-started.md:87, the only
marketplace.visualstudio.com URL in the tree. The link is not broken for a human — the Marketplace
returns 503 to automated requests, the same bot-blocking .lychee.toml already carries exclusions
for on apps.microsoft.com/store/detail/ (403), code.visualstudio.com (403), stackoverflow.com
(403) and olcf.ornl.gov/summit (503). accept lists 502 and 504 but not 503, so it fails hard
after all 5 retries and takes the job with it.

Excluding the host is the targeted fix; adding "503" to accept would suppress genuinely dead
links everywhere else.

Type of change

• Other (CI configuration)

Testing

• .lychee.toml parses, and the new pattern matches the failing URL.
• Checked it does not over-match: visualstudio.com, github.com and mflowcode.github.io links
  are still checked.
• ./mfc.sh precheck passes all seven gates.

Checklist

• I added or updated tests for new behavior
• I updated documentation if user-facing behavior changed

[Copilot]

Pull request overview

Warning

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Updates Lychee link-check configuration to avoid CI failures caused by VS Code Marketplace returning 503 to automated requests.

Changes:

• Add marketplace.visualstudio.com to Lychee’s exclude list to prevent repeated 503 failures during link checking.

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