docs: add Sourcey-generated API site

[bbbbzzzzcc-afk]

Add a reproducible Sourcey documentation site

Summary

• add a Sourcey-generated documentation site for ftpserverlib, pinned to commit b4c3694ee73399d8a55293d568e5100c25e4d2d4
• document 92 public Go API concepts plus five maintained guides
• add deterministic validation, internal-link checks, and verification of the committed production-signed RunX receipt
• build on pull requests and deploy the same validated output to this repository's GitHub Pages site after merge

Why

The project README gives a useful overview, but consumers currently have to navigate source and GoDoc to understand the driver contract, settings, and extension points. This adds a browsable project-owned reference with pinned source links and machine-readable llms.txt outputs.

Verification

npm ci
npm run test-linkcheck
npm run build
npm run validate
npm run linkcheck
npm run verify-receipt

All commands pass locally. The generated snapshot reports 92 public API concepts, 9 HTML pages (including the compatibility alias), 65 pinned source links, and 0 broken internal references.

Maintainer action after merge

If GitHub Pages has not been enabled for this repository before, select GitHub Actions as the Pages source once. The workflow then deploys docs-sourcey/dist to the repository-owned Pages site.

No library runtime code or Go dependencies are changed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.48%. Comparing base (b4c3694) to head (3aafb32).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #656   +/-   ##
=======================================
  Coverage   87.48%   87.48%           
=======================================
  Files          13       13           
  Lines        2102     2102           
=======================================
  Hits         1839     1839           
  Misses        175      175           
  Partials       88       88           

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