fix: close post-1.0 review defects

[Vonng]

Summary

This is the minimal, evidence-backed repair set from the post-1.0 adversarial review:

• make cached sidebars page-independent, visible without JavaScript, correctly hydrated, and free of transient contrast failures;
• serialize plain and whole-Book Print variants per page, remove Print shell chrome, and restore the book-toc invalid-input fallback;
• normalize Redoc static-root URLs, stop invalid Landing preview input before string operations, and retire the no-op ScrollSpy chunk without breaking 1.x configuration;
• remove only confirmed dead Store/i18n/checker state;
• make rendered-output checks fresh by default and bound every checker-owned Hugo process;
• reduce duplicate strict builds while retaining every per-case warning/fallback/output assertion;
• make sibling validation commands path-independent.

The checker trace changed from 735 Hugo calls / 305 strict / 34 bounded / 322s to 614 / 184 / 613-of-613 bounded / 275s. No arbitrary duration gate or testing framework was added.

The stable v1.0.0 release branch repair was completed separately in #33; this PR does not move tags or release refs.

Companion EN/ZH contracts, guides, site tests, and cleanup are in the documentation-site PR.

Scope guard

This does not prune Bootstrap, collapse the supported search providers, remove non-English fallback locales, remove compatibility partials, or retire migration tools.

Verification

• full ordered theme checker suite
• strict Hugo fixture
• 85 migration tests
• 3 checker-input tests
• 41 browser-runtime unit tests
• output/namespace/schema/goldens
• sibling site non-browser suite
• Playwright: a11y 30/30, responsive 45/45, keyboard 16/16, content 10/10, code 18/18, Landing 4/4, theme color 5/5

Closes #24
Closes #25
Closes #26
Closes #27
Closes #28
Closes #29
Closes #30
Closes #31
Closes #32
Closes #34
Closes #35

[Vonng]

Companion EN/ZH contracts and real-site regression PR: pgsty/oink.pgsty.com#9. Both PR check suites are green; local candidate validation used the sibling theme replacement.