docs(be,fe): design doc and implementation checklist for verified emails as a first-class II primitive

[MRmarioruci]

Internet Identity has a single recovery-email slot per anchor, verified through an inbound DKIM/DMARC/DNSSEC ownership-proof flow. Dapps can request email / name / verified_email attributes but only from linked OpenID/SSO sources — the recovery email is invisible to the attribute system. Users can't add multiple verified emails for any other purpose, and there's no way to share an email with a dapp that doesn't go through a third-party identity provider.

This PR adds two design artifacts to start the conversation; no code changes.

Changes

Two new files under docs/design/:

• verified-email-attributes.md — design doc. Proposes promoting "verified emails" to a first-class anchor primitive that supersedes the current single-slot recovery model, with three phases:
  • Phase 1: verified-emails primitive + batched migration (1a lazy on touch, 1b controller-driven sweep of the long tail, 1c drop the legacy field).
  • Phase 2: verified emails as ICRC-3 attribute sources under a new verified:<H(address)>:email scope.
  • Phase 3: smart-routing + last-used default in the consent dialog; no per-dapp pin, no global "don't share" toggle.
• verified-email-implementation.md — operational checklist. File-path-level steps for implementers, with locked decisions stated up front (cap=5, wizard rename to emailVerification/, additive candid evolution, etc.) and a suggested PR breakdown for the eventual feature work.

Both files cross-link to each other. The design doc explains why; the checklist is what to do.