fix(landing): the front door matches the house — content visible, dark real, mobile whole #5541

Design Wave A, part 2 — and two sweep findings corrected by root-causing (agent-built, render-proven)

1. The "dead sections" were never empty. All seven carried complete, designed content hidden behind .lp-reveal{opacity:0} awaiting an IntersectionObserver — invisible to no-JS visitors, reader modes, and every full-page capture. The reveal pattern is deleted outright with a comment banning it: content must not require JavaScript to exist. Two genuine intra-page duplicates (Enterprise-auth card, MCP card duplicating the dedicated section) were removed under doctrine.
2. Dark mode is real: data-theme="light" was hardcoded with literal-only CSS. The / handler now resolves the theme cookie; every literal became a token (52 → 18, remainder deliberate: fixed navy band, traffic-light dots, meta theme-colors). Evergreen replaces stray indigos.
3. The "mobile duplication" was a capture artifact + one real bug: the page's 18,520px phone height exceeds Chromium's 16,384px max texture, so fullPage captures rasterized the tail as garbage (proven both directions with test pages); the one real defect was the prelaunch pill overflowing the 390px nav. Nav compaction + phone density → 15,240px, inside the rasterizer, no horizontal scroll.

Typecheck clean · 96 landing-adjacent tests green · design-audit zero banned · screenshot-verified 1440-light/1440-dark/390 against a local server.

Sweep-tooling note carried forward: any page >16,384px corrupts in fullPage capture (gates at ~17.3k is affected) — a stitched-scroll mode for design-render-sweep.mjs is queued, and some very-tall-page findings deserve re-verification after it.

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