test(intake): e2e top-ups — SHINES-5xx form error + dup-kid 409 + secure-context doc (coverage W5)

Summary

Wave 5: thin e2e top-ups + document the browser-keygen secure-context limit. Depends on Wave 0 (devstack-reachable fault toggle).

Acceptance criteria

• e2e: SHINES-mock-5xx → the form surfaces a redacted error (no leak).
• e2e: duplicate-kid registration → 409 at /keygen.
• Documented: crypto.subtle secure-context limitation (spec comment + a doc line).
• cargo xtask e2e --no-refresh -- --project=intake-ui-shines green.

References

Plan §Wave 5.