Route Guide three-categories cards to canonical goal copy

[PunchTheDev]

Summary

Guide's The three categories section was using invented prose (e.g. "Lightest part that passes FEA. Material density and topology matter most.") that didn't match the canonical one-liner goal strings used on the category-page H1 (App.tsx:CATEGORY_META) and spec-detail CATEGORY_PILL.goal row.

Changes

• CATEGORIES rewritten to lead with canonical goal strings (verified character-exact against App.tsx:90,99,108): "Lightest part that survives the load wins." / "Highest stiffness-per-gram wins." / "Least bending under the applied load wins."
• Each card is now a routed <Link to="/problems/round_NNN"> with hover state + Open category → micro-CTA — first-timer can click straight to the 15 live problems.
• Unit chips become cursor-help ↓ minimize g / ↑ maximize N/(mm·g) / ↓ minimize mm with 154/162/179-char tooltips defining each unit and describing how the FEA harness computes the score.
• "easy / medium / hard" tier line wrapped in 191-char cursor-help tooltip reusing the canonical wording from App.tsx:711 (step 354 — same explanation appears on the Category-page filter row).
• BACKLOG.md Guide → The three categories flipped ○ ○ ○ → ● ● ●.

Verification

Puppeteer-verified live at 1440×900 on /guide#categories: 3 routed cards · all 3 canonical goal strings present · old "Lightest part that passes FEA" copy gone · tier tooltip 191 chars · click on Mass card navigates to /problems/round_001. 0 new console errors.