feat(web): refine project proof grid and scroll reveal

[daguanren21]

Problem / Goal

The production-baseline homepage had a proof section that rendered real project evidence as an undifferentiated vertical stack, while the first scroll reveal was too subtle to observe reliably. Make the evidence scannable and make the reveal visibly communicate project proof without replacing the existing cosmic hero.

Change

• Apply a visible, reduced-motion-aware scroll reveal to the three real project screenshots already embedded in the project rows.
• Rebuild the engineering proof area as responsive comparison cards: 3 columns on wide screens, 2 on medium screens, 1 only on narrow phones.
• Add project identity, evidence labels, PROOF markers, bounded/wrapped code panes, and localized BUILD LENS heading.
• Preserve existing project links, copy interaction, bilingual output, particle/orbit hero, static output, and reduced-motion behavior.

Non-goals

• No standalone decorative MacBook section.
• No replacement of the production particle/orbit hero.
• No dependency or runtime animation library.
• No merge, production deployment, or unrelated content/data changes.

Verification

• pnpm --filter @weapp.dev/web check — 0 errors, warnings, or hints.
• pnpm --filter @weapp.dev/web lint:styles — pass.
• pnpm --filter @weapp.dev/web build — 28 pages; 29 required outputs/internal links validated.
• Focused Playwright desktop/mobile landing matrix — 14 passed, including initial/runtime reduced motion, proof scroll, responsive 3/2/1 columns, overflow, and key routes.
• Adjacent desktop route/asset checks — 6 passed.
• git diff --check — pass.

Compatibility / Risk

The scroll synchronizer is inline and dependency-free, throttled to one update per 16ms, and fully disabled/reset when reduced motion is enabled. Existing static content remains available without JavaScript. Responsive breakpoints are covered by browser assertions.

Rollback

Revert the single commit on this PR. No data migration or deployment-side state is introduced.

Issue linkage

None.