docs: uniform 16:9 gallery heroes + self-sufficient nav

[TMHSDigital]

Phase 2 of the gallery review/facelift, driven by headless-browser screenshots (Phase 0 capability stood up: puppeteer-core + installed Chrome, captured desktop+mobile × light+dark).

What the screenshots showed (Phase 1)

• [LOCAL-FIXABLE] Ragged desktop 2-col layout: the swatch hero was 8:3 panoramic while turntable/gn-sdf were 16:9, so the grid row heights were uneven and a gap sat under the short card. Swatch spheres were also left-clustered with empty frame on the right.
• Contrast, top bar, mobile density, theme toggle all read clean in both themes.
• [TEMPLATE-BLOCKED] landing→gallery nav link — recorded, not attempted (fleet template owns the landing nav).

Fixes (this repo's owned files only)

• Re-rendered the swatch hero at 16:9 (1280×720) — 6 spheres now fill the frame; verified PASS (6 distinct regions, non-black, correct EEVEE id).
• Gallery CSS: uniform 16:9 media (object-fit: cover, no crop now that heroes match), align-items: stretch, bottom-pinned "View example" so paired cards are equal height with aligned footers.
• GitHub repo link added to the top bar → the gallery is self-sufficient for navigation (back-link to landing already present).
• DESIGN_NOTES.md updated with the evolved direction (supersedes the mixed-aspect approach).

Verification

• Local file:// screenshots (desktop+mobile, both themes) confirm the gap is gone and cards are uniform.
• Live deployed screenshots pasted after merge (Pages deploys on push to main, not PR).
• README counts (12/6/2/17) untouched; gallery.json schema unchanged.

🤖 Generated with Claude Code