Fix multilingual SEO discovery

[SingLinkLabs]

Summary

• publish all English, Simplified Chinese, and Traditional Chinese canonical pages in the XML sitemap
• add bidirectional en, zh-Hans, zh-Hant, and x-default hreflang annotations
• output correct document language and localized descriptions for Chinese landing pages
• remove duplicate page-level H1 headings without changing URLs or article content
• add source and rendered-site SEO validation to GitHub Actions

Root cause

The original static sitemap listed only six English URLs. Chinese pages were crawlable, but they were not listed in the sitemap, had no hreflang relationship, and rendered with the global English document language.

Validation

• built locally with ghcr.io/actions/jekyll-build-pages:v1.0.13
• validated 18 rendered canonical pages
• sitemap contains 18 canonical URLs and 72 hreflang annotations
• every mapped page has one H1, a self-canonical, a localized description, and no noindex
• publication guard passed for 47 public files
• 4 unit tests passed
• 62 repository-local links passed

Scope

Public documentation, metadata, templates, and CI validation only. No product source code, credentials, node configuration, payment code, or backend logic is included.