feat(sdk): users::email_available for anonymous registration pre-check (S1)

`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>

crates/manager-core/src/realtime_authority.rs

@@ -398,6 +398,13 @@ mod tests {
         ) -> Result<Option<picloud_shared::AppUser>, picloud_shared::UsersError> {
             unimplemented!()
         }
+        async fn email_available(
+            &self,
+            _: &picloud_shared::SdkCallCx,
+            _: &str,
+        ) -> Result<bool, picloud_shared::UsersError> {
+            unimplemented!()
+        }
         async fn update(
             &self,
             _: &picloud_shared::SdkCallCx,