Inline N alongside kg + spell out Safety factor on spec card

[PunchTheDev]

Summary

Spec spec-card BACKLOG item flagged two first-timer friction points:

1. Units (N vs kg) repeated — chip said 15 kg load while the SpecDiagram (step 379) labels the arrow ↓ 149N. First-timer reads both and wonders "wait, is it 15 or 149?"
2. "SF 1.5×" cryptic — same abbreviation PR Spell out SF as Safety factor in submission journey #280 already fixed in submission journey

Changes

• HeroStats.tsx L168-172: chip rebuilt as `15 kg (149 N) load` with the N in muted text so the kg framing stays primary; tooltip explains "149 N is the force the FEA solver applies; 15 kg is the everyday equivalent (1 kg ≈ 9.81 N)" so the two numbers explicitly cross-reference instead of silently disagreeing
• HeroStats.tsx L173-177: `SF {n}×` → `Safety factor {n}×`; tooltip resolves the abstract multiplier to the concrete `{n} MPa` allowable stress (per step 280 spell-out pattern, no abbreviations on first-contact UI)
• HeroStats.tsx L203-209: plain-English paragraph gains `(149 N)` after `15 kg` so the cross-unit reads identically in prose

Verify

Puppeteer 1440×900 on `/problems/round_001/r01_001_easy`:

• loadChip text: `15 kg (149 N) loadⓘ` · tooltip 254 chars
• sfChip text: `Safety factor 1.5×ⓘ` · tooltip 152 chars
• plainEnglish text: `...must hold 15 kg (149 N) pulling straight down...`
• no `SF \d` text rendered anywhere on the page
• 0 new console errors (1 pre-existing 404 unchanged)