`users::find_by_email` requires an authenticated principal (F-S-003,
anti-enumeration), so the natural check-then-create register pattern 502s
for anonymous public scripts. Add `users::email_available(email) -> bool`,
gated like `create` (the registration write path) but without the
anonymous rejection: it leaks only a single boolean — no more than
`create`'s own uniqueness error already does — so self-serve register
scripts can pre-check. Also document find_by_email's principal
requirement in the SDK doc-comment.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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