fix: finish Markmap, footer clearance and technical table repairs

[Vonng]

Keep Markmap labels readable across appearance changes, clear fixed right rails above the footer after both scrolling and content reflow, and provide explicit .td-table--wide sizing for technical tables while preserving ordinary-table layout.

Includes the original commits from #48, #50 and #54, plus main-content ResizeObserver coverage, opt-in table scoping, fixture output checks and reviewed goldens. Browser geometry tests use the companion site’s existing Playwright suite. FileTree #52 and PlantUML #46 remain deferred. The Hugo 0.168 / Dart Sass prerequisite is #55.

Validation:

• 73 JavaScript runtime tests and 52 output goldens pass; output integrity, namespace, shell, content, agent-index and backlink checks pass.
• Companion site: make check passes all 60 tests plus bilingual/link checks; make browser passes all 365 tests, including the 421-page accessibility scan.
• 27 focused Chromium/Firefox/WebKit cases cover English/Chinese Markmap, footer reflow, technical tables, mobile, keyboard and print.
• Strict production build passes; an injected Sass @warn still fails --panicOnWarning.
• Local tools: Hugo Extended 0.168.0, standalone Dart Sass 1.105.2, Go 1.27.2, Node 26.12.0. CI pins Go 1.27.0. Final candidate CI: https://github.com/pgsty/oink/actions/runs/39042881613

Full browser acceptance used the final runtime code. Later changes only add release notes and distinguish a table fixture ID from its generated heading ID; fresh final output checks and goldens also pass. The companion contracts/tests are prepared in pgsty/oink.pgsty.com#11, which remains draft until the next published theme version is pinned. No new release or deployment is included.

Use a merge commit, preserving contributor history and the original PR ancestry.

Fixes #47
Fixes #49
Fixes #53