fix(perry/ios): perry-ui-ios fails to compile — stale perry_runtime::thread::pin_promise reference #9961

perry-ui-ios fails to compile for an iOS target. foundation_models.rs:108 calls perry_runtime::thread::pin_promise, which no longer exists — it is the only reference to that symbol left in the tree, and it was orphaned when the pinning API moved (last touched by #8700 in August).

error[E0425]: cannot find function `pin_promise` in module `perry_runtime::thread`
   --> crates/perry-ui-ios/src/foundation_models.rs:108:37
error: could not compile `perry-ui-ios` (lib) due to 1 previous error

Why it went unnoticed: the crate is #![cfg(target_os = "ios")], so on a macOS host cargo build -p perry-ui-ios compiles it to nothing and the reference is never checked. It only surfaces on an actual iOS target build (--target aarch64-apple-ios-sim).

Fix: pin the cross-thread promise through the current API. The primitive is perry_runtime::gc::pin_object on the promise's GC header; perry_stdlib::common::async_bridge::pin_promise_for_native_resolution(promise_ptr: usize) already wraps exactly this. Route foundation_models.rs to the same mechanism.

CI gap to reconcile: iOS workflows exist (test.yml, simctl-tests.yml). If none of them compiles perry-ui-ios for an iOS target, that is the gate gap that let this land — a compile of this crate for aarch64-apple-ios[-sim] would have caught it. Worth a required build step.

Found while implementing #9954 (TextField padding), which needed an iOS-sim compile to verify.

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