Fix lychee CI link checker failures

[kixelated]

Summary

• Add --fallback-extensions md to the doc lychee command so VitePress-style links (without .md extension) resolve correctly
• Accept 403 status codes from sites that block automated requests (npmjs.com, linode.com)
• Exclude nixos.org and letsencrypt.org which have intermittent network failures in CI

Test plan

• Verified lychee --root-dir "$(pwd)/doc" --fallback-extensions md doc passes locally with 0 errors
• CI passes

🤖 Generated with Claude Code

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Walkthrough

The PR updates link-checking and CI. The justfile changes lychee calls to add --fallback-extensions md for the initial doc scan and to exclude CHANGELOG\.md in the second call. lychee.toml updates exclude_path (escaping CHANGELOG\.md and adding demo/web/src/.*\.html), adds accept for HTTP 200 and 403, and adds an exclude list for three external URLs. The GitHub Actions check job adds an early free-disk-space step. Multiple README files have updated hyperlink targets and relative license/doc paths.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and specifically describes the main focus of the pull request - fixing lychee CI link checker failures.
Description check	✅ Passed	The description clearly explains the key changes made to fix lychee link checking issues, including fallback extensions, HTTP status handling, and exclusions.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-lychee-ci

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch fix-lychee-ci

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