Reach 100% Ruby coverage and add browser regression tests

[samuel-williams-shopify]

This closes the missing Ruby and template coverage with fixture-backed tests for guide ordering and navigation, reference pages, supplemental Markdown, releases, fallbacks, and exported Bake tasks. Coverage now includes unloaded task/controller files too, with no new exclusions.

The tests also expose and fix asymmetric guide ordering (missing orders now default to zero, with names breaking ties), supplemental documentation resolving against the working directory, missing guides/references returning errors instead of 404s, and crashes with empty READMEs, empty Markdown headings, or guides without introductions. Empty headings use the existing title fallbacks and preserve following content in rendering and documentation tasks.

Coverage	Before	After
Measured Ruby/template line coverage	819/911 (89.9%)	1,054/1,054 (100%)
Ruby tests	24	70
Browser cases in CI	0	24

The new Chromium suite checks mobile/desktop layouts in light/dark mode, local links and decoded fragment targets under a GitHub Pages subpath, sidebar/deep-link behavior, Mermaid and syntax highlighting, keyboard disclosures, table spacing/overflow, and successful/unavailable search. It builds a real fixture site and reuses the documentation workflow's pinned Pagefind fork. Browser checks are behavioral tests, not a claim of 100% JavaScript coverage.

Validation:

• Clean Ruby coverage run: 70 tests, 197 assertions, 100% measured line coverage; covered:validate passes.
• RuboCop, documentation coverage (96/96 public definitions), and web package checks pass.
• All 24 browser cases pass in CI, including real Pagefind search and fragment validation. The link check also correctly rejects a deliberately removed cross-page method anchor.