fix(runtime): structuredClone keeps overflow/dynamic properties (#4879)

[proggeramlug]

Fixes #4879.

Root cause

Exactly the suspected mechanism: the GC_TYPE_OBJECT branch of js_structured_clone_inner (builtins/globals.rs) clones via js_object_clone_with_extra, which copies only the inline field_count slots and truncates the keys array to min(keys_len, field_count). Anything in the OVERFLOW_FIELDS side table — or every property of a {}-born object, which has no inline capacity at all (explains the issue's "even f0 is undefined at 50 props" wrinkle: field_count is 0 there, not 8) — vanished from the clone.

Fix

When keys_array.length > field_count, rebuild the clone key-by-key: js_object_get_field resolves inline-vs-overflow per index, values recurse through js_structured_clone, and js_object_set_field_by_name writes into a fresh object. RuntimeHandleScope roots src/clone across the recursive deep-clone (it can trigger minor GCs), and key bytes are owned before recursing. Fully-inline objects keep the existing clone_with_extra fast path untouched.

Validation

Perry output now matches node --experimental-strip-types exactly on:

• the issue repro (50 dynamic props: f0/f49/key count all correct)
• 3000 dynamic props: 0 missing (was 2992)
• literal round-trip + deep-copy independence (mutating clone leaves source alone)
• small {}-born objects, nested dynamic-object-inside-array-inside-literal
• New unit test structured_clone_keeps_overflow_properties (50-prop {}-born object); the existing structured-clone GC rooting test still passes.

Code-only PR — version bump + changelog left for merge time.