perry-ui-* callback tables hold NaN-boxed JS closures with no GC scanner

[proggeramlug]

Every perry-ui-* crate caches JS callbacks as raw NaN-boxed f64 in thread-local side tables, and no UI crate registers a gc_register_mutable_root_scanner. The ledger currently carries 466 perry-ui-* frontier entries of this shape.

Representative instances (all the same pattern):

• crates/perry-ui-ios/src/adaptive_layout.rs: LISTENERS: RefCell<HashMap<i64, f64>> (added in feat(ios): add iOS 27 platform APIs #8699)
• crates/perry-ui-ios/src/network.rs: LISTENERS: RefCell<HashMap<i64, f64>>
• crates/perry-ui-macos/src/network.rs: LISTENERS: RefCell<HashMap<i64, f64>>
• crates/perry-ui-macos/src/state.rs: ON_CHANGE_CALLBACKS: RefCell<HashMap<i64, Vec<f64>>>
• crates/perry-ui-ios/src/state.rs: ON_CHANGE_CALLBACKS: RefCell<HashMap<i64, Vec<f64>>>
• crates/perry-ui-macos/src/widgets/mod.rs: HOVER_CALLBACKS: RefCell<HashMap<i64, f64>>

Why this matters

A stored callback is a heap pointer held across arbitrary time, including across collections. Two independent failure modes:

1. Not marked — the collector can reclaim a listener that is still reachable only from the UI table, so a later invoke_listener calls freed memory.
2. Not rewritten — after an evacuating minor the cached pointer still points into from-space. Per docs/src/internals/gc-rooting-invariant.md this surfaces cycles later as TypeError: value is not a function, far from the cause.

This is the class CLAUDE.md describes as invisible to the static checker: scripts/gc_root_dominance_check.py reads emitted LLVM IR, so a side table holding a heap pointer is structurally out of its reach. The runtime instruments (PERRY_GC_PROTECT_FROMSPACE, PERRY_GC_SCHEDULE_SEED) are the only detectors.

Note the reproducibility tell from the same doc: an unrooted table goes bad at collection #0 and stays bad, unlike an unrooted register which is intermittent.

Suggested shape

Register one scanner per UI crate covering its callback tables, in the manner of gc/roots.rs's existing gc_register_mutable_root_scanner users, then move the corresponding frontier entries to holders with a real verdict. The frontier entries are the work list.

Found while auditing #8699; not caused by it.