Iter 4: result snapshot bugfix, favicon, /stats dashboard, /how page ...

- BUGFIX: /api/result now returns categories/calibrated/total_time_s —
  score snapshot stored in new score_json column (auto-migration),
  old rows degrade gracefully
- favicon.svg (brutalist V mark) linked on all pages
- /stats: live dashboard (starts, signed results, avg IQ, integrity line)
- /how: 'How verification works' — Ed25519 explainer for skeptics with
  offline-verify snippet and honest-limits section
- live-verified: all pages 200, full cycle + OG PNG + scoring tests pass

VerIQ v1 MVP: FastAPI backend, 40-task bank, frontend, Ed25519 signing ...

- backend: main.py (API + sessions + rate limit), scoring.py (weighted
  z-score normalization, sanity checks), signing.py (Ed25519 canonical
  payload signing), db.py (SQLite WAL, results + events)
- content: questions.json v1.0.0 (40 original tasks, 4 categories,
  SVG figures), calibration.json placeholder
- frontend: brutalist dark theme, test engine with timer/resume,
  result page with share links, public verify page
- docs: SPEC.md (product + tech spec, milestones)
- smoke-tested: full cycle start->finish->verify, tamper rejection OK