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/executor-core/src/sdk/users.rs

@@ -4,7 +4,12 @@
 //! // CRUD
 //! let u   = users::create(#{ email: "a@b", password: "hunter22a", display_name: "Alice" });
 //! let u   = users::get(id);                      // map or ()
-//! let u   = users::find_by_email("a@b");         // map or ()
+//! let u   = users::find_by_email("a@b");         // map or () — REQUIRES an
+//!                                                //   authenticated principal
+//!                                                //   (anti-enumeration); forbidden
+//!                                                //   for anonymous public scripts.
+//! let free = users::email_available("a@b");      // bool — anonymous-safe pre-check
+//!                                                //   for self-serve registration.
 //! users::update(id, #{ display_name: "Alicia" });
 //! let removed = users::delete(id);               // bool
 //! let page = users::list(#{ "$limit": 50, cursor: () });
@@ -56,6 +61,7 @@ pub(super) fn register(engine: &mut RhaiEngine, services: &Services, cx: Arc<Sdk
     bind_create(&mut module, &svc, &cx);
     bind_get(&mut module, &svc, &cx);
     bind_find_by_email(&mut module, &svc, &cx);
+    bind_email_available(&mut module, &svc, &cx);
     bind_update(&mut module, &svc, &cx);
     bind_delete(&mut module, &svc, &cx);
     bind_list(&mut module, &svc, &cx);
@@ -136,6 +142,27 @@ fn bind_find_by_email(module: &mut Module, svc: &Arc<dyn UsersService>, cx: &Arc
     );
 }
 
+/// `users::email_available(email)` → `bool`. Anonymous-safe pre-check for
+/// self-serve registration (unlike `find_by_email`, which forbids
+/// anonymous callers). Lets a public register script branch on a free vs
+/// taken email without relying on `create`'s uniqueness error/502.
+fn bind_email_available(module: &mut Module, svc: &Arc<dyn UsersService>, cx: &Arc<SdkCallCx>) {
+    let svc = svc.clone();
+    let cx = cx.clone();
+    module.set_native_fn(
+        "email_available",
+        move |email: &str| -> Result<bool, Box<EvalAltResult>> {
+            let email = email.to_string();
+            let svc = svc.clone();
+            let cx = cx.clone();
+            block_on(
+                "users",
+                async move { svc.email_available(&cx, &email).await },
+            )
+        },
+    );
+}
+
 fn bind_update(module: &mut Module, svc: &Arc<dyn UsersService>, cx: &Arc<SdkCallCx>) {
     let svc = svc.clone();
     let cx = cx.clone();