avm2: Check prototype chain in with-scope property resolution

[bjornwein]

Problem

When AS3 code uses XML methods like name(), children(), etc. inside a with block (pushwith opcode), the compiler generates QName(PackageNamespace(""), "name") lookups (public namespace). However, Ruffle's search_scope_stack only checked:

1. vtable traits — XML's vtable has name() in the AS3 namespace (http://adobe.com/AS3/2006/builtin), not the public namespace → no match
2. own properties (for with-scopes) — XML's has_own_property checks E4X child elements → no match

This caused findpropstrict to skip the XML with-scope entirely and find the property on a parent scope instead (e.g., DisplayObject.name getter returning a string), leading to Error #1006: value is not a function when trying to call the string result.

Root Cause

In avmplus, hasMultinameProperty for with-scope resolution checks own properties, traits, and the prototype chain (delegate). Ruffle was missing the prototype chain check. This matters because XML.prototype.name (and other XML methods) are defined as public-namespace dynamic properties on the prototype in XML.as (lines 183-190), bridging the AS3→public namespace gap.

Fix

Added value_has_proto_property() to search_scope_stack that walks the prototype chain checking for dynamic properties. This is called for with-scopes after the existing own-property check, matching avmplus behavior.

Testing

Tested against a real-world Flex-based SWF (hardware appliance UI) that was completely broken by this bug — the app now loads and functions correctly past login. The Error #1006 in updateTreeExpansion() is eliminated.

Existing tests pass (cargo test --release --package ruffle_core).

[evilpie]

Please add a test, thank you.

[bjornwein]

Thanks for looking into it. I apologize for dropping this LLM-generated PR on you.

I can confirm that the change solves a real-world problem for me. Unfortunately I don't have the knowledge to evaluate whether this is the best way to solve the specific issue.
I hope that your review will be able to spot any obvious significant performance costs or regression risks.

I (or mostly copilot) have updated the PR with a minimal-ish regression test. The test fails with error code #1065 without the fix.