Proposal 04: add the WebAuthn autocomplete token

Design sketch only. Confirm the current login input and form ownership at the target core ref.

Goal

Conditional passkey UI requires the username input used by the active login form to include the webauthn autocomplete token:

<input autocomplete="username webauthn">

Checklist

• Apply the token to the real username field in every active login markup generation.
• Preserve existing username autofill and accessibility semantics.
• Confirm browsers without conditional mediation ignore the extra token safely.
• Test rendered HTML and a real conditional-UI ceremony when the native bundle is armed.
• Do not treat this token as native passkey support by itself; without the server ceremony, exact-origin policy, and login integration it is inert.

Whether this lands independently or with the native login surface is a maintainer/release decision, not a compatibility claim in this repository.