Redesign the compare report as a single-page triage UI

[jeremyfelt]

Replaces the flat per-comparison report pages with one static reports/index.html that embeds every result as JSON (window.REGLANCE) and renders three views client-side from the location hash — no framework, no network access (opens straight from disk).

Views

• Overview — results grouped by page, changed-only by default, diff thumbnails, +added −removed HTML counts, search, and full keyboard triage (worst-first j/k queue, Enter to compare, space quick look, h HTML diff, ? shortcuts).
• Comparison — swipe, side-by-side, onion skin, diff overlay, and blink modes.
• HTML diff — unified diff with line numbers, hunk headers, and collapsed unchanged regions.

Auto light/dark via CSS light-dark(); the "Quiet" design direction from the handoff.

Generator changes

• report.mjs: new buildHtmlDiff() emits changed-line counts and unified-diff hunk JSON (replacing the boolean Yes/No signal); generateReport() groups per-slug results into one entry per page and writes the single page with run metadata (compared/baseline timestamps, duration).
• compare.mjs: compareSlug() returns HTML add/del/hunks inline and no longer writes per-result report or html-diff files; compare() gathers run metadata and builds the single report.

Templates

New index.html shell plus assets/reglance.css and assets/app.js (vanilla renderer). Removes the old report.html, the index table, diff-viewer.html, html-diff.html, and their stylesheets/script.

Notes

• Data-derived values (page keys, URLs, raw HTML source) are rendered via textContent, and the embedded JSON neutralizes </script> — verified by tests and a headless-browser smoke check.
• Tests rewritten for the new API (npm test → 111 passing); npm run lint clean. eslint ignores the design-handoff prototypes.
• The design_handoff_reglance_report/ reference bundle is intentionally not included in this branch.

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