docs: publish Sourcey-generated Go reference

[zdfgu113]

Summary

• keep the existing MkDocs site and theme unchanged
• publish a Sourcey-generated Go reference under /reference/
• generate package documentation from the checked-out Go source during the existing Pages workflow
• link the generated reference from the MkDocs navigation

Rationale

The current guides remain the best entry point for users, while the generated reference gives maintainers and consumers a searchable view of exported packages and symbols that stays synchronized with the source. No generated HTML is committed.

Sourcey is pinned to 3.6.5, and the build runs from the same checkout that Pages deploys. The small compatibility copy keeps Sourcey's Go API tab route working with its default flat HTML output.

Validation

• npx --yes sourcey@3.6.5 build --output site/reference
• 9 Sourcey-rendered pages generated from the existing MkDocs content and live godoc adapter
• browser-checked the reference home, Getting Started, CLI Reference, Go Library, Go API overview, and package pages under /reference/
• git diff --check

go test ./... still has the repository's existing Windows-only failures where DNS-flush tests stub the Unix true command; this change does not modify Go code.

[zdfgu113]

A public preview of the proposed Sourcey reference is available at https://zdfgu113.github.io/txeh/reference/. I also ran a read-only validation against the PR checkout: Sourcey 3.6.5 markers were present on 10 pages, the godoc snapshot contained 82 public API items, and browser checks covered guide navigation, the Go API route, search, and a search-result deep link. The PR remains mergeable and does not commit generated HTML.