Cache the flattened enumerable key snapshot for each `for..in` site and
reuse a `PropertyNameIterator` when the receiver shape, dictionary
generation, indexed storage kind and length, prototype chain
validity, and magical-length state still match.
Handle packed indexed receivers as well as plain named-property
objects. Teach `ObjectPropertyIteratorNext` in `asmint.asm` to return
cached property values directly and to fall back to the slow iterator
logic when any guard fails.
Treat arrays' hidden non-enumerable `length` property as a visited
name for for-in shadowing, and include the receiver's magical-length
state in the cache key so arrays and plain objects do not share
snapshots.
Add `test-js` and `test-js-bytecode` coverage for mixed numeric and
named keys, packed receiver transitions, re-entry, iterator reuse, GC
retention, array length shadowing, and same-site cache reuse.
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