docs: add InfraLens migration parity report (Phase 2)

- docs/infralens/MIGRATION_PARITY.md — 24-point source/target comparison
  per the migration plan §54, verdict: full functional parity except the
  deliberately-dropped PWA
- validated via fresh lint/typecheck/build/282 unit tests/40 e2e
  (portfolio+infralens), plus a manual browser pass (history, why-score
  dialog, results filters, SSRF-blocked target, DNS-failure target,
  docs/privacy/compare pages, mobile viewport)
- flags one pre-existing, non-migration gap: Next.js's production Server
  Action error redaction swallows InfraLens's intended user-facing
  security messages (localhost/invalid-target rejections show a generic
  Next.js message instead of the app's own) — not introduced here, the
  source repo's own e2e always ran against pnpm dev, never pnpm start,
  so this was already true in production on infralens.dev

Author: Randy-R-code