perf: plain struct values and cached namespace lookups (classic)

[triniwiz]

Summary

Two changes to the classic (V8) engine.

1. Struct values are plain JS objects

WinRT structs were interceptor objects. Each one built a new ObjectTemplate and boxed a DeclarationFFI that was never freed. Now each struct type gets one FunctionTemplate per isolate (runtime/src/struct_value.rs), and values are ordinary objects with one property per field. All values of a type share one V8 map and the constructor's prototype.

WinRT structs are value types, so a value is a snapshot. Passing it back to native serializes its fields with the existing append_struct_object_bytes. DeclarationFFI.struct_instance is removed.

Bugs fixed along the way:

• About 550 bytes leaked for every struct returned from native.
• Every new Rect(...) wrote into one buffer stored on the constructor, so creating a second struct changed the first.
• Nested struct fields read as undefined (e.g. Duration.TimeSpan).
• {...s} gave {}, JSON.stringify(new Rect(...)) gave {}, String(s) threw, instanceof was false, and properties added in JS were dropped.
• new Rect() threw, and new Rect({ X: 1 }) produced a broken object. Missing fields are now zero, and wrong field types throw a TypeError.

A TypeName value ({ Name, Kind }) passed to a method or property is serialized, and String struct fields are now written.

2. Namespace and enum lookups are cached as real properties

The namespace/enum interceptor is now non-masking. Each resolved name is defined as a read-only, non-enumerable own property, so repeat lookups no longer go through the interceptor. Writes to a reserved name are ignored, and throw a TypeError in strict mode, same as an already-resolved name.

Gotcha: define_own_property goes through the object's setter interceptor. A thread-local guard lets the define through, and the side store keeps a copy as a backup. Without that guard, every lookup rebuilt a constructor and a V8 template, which are never freed, and memory grew without limit. namespace_members.rs checks Windows.Foundation.Uri === Windows.Foundation.Uri to catch this.

Performance

Release build, ns per op over 1M iterations:

Access	Before	After
Windows.UI.Colors.Red.R	~3100	~760
read a field of a struct you already hold	55	~1
memory held after 1M struct returns	+540 MB, never freed	+0 MB
Windows.Foundation.Uri	159	8
Windows.UI.Xaml.Controls.Button	355	27

Behaviour changes

• Struct values are snapshots. A struct read from a property doesn't track later native changes, which was already true.
• Object.keys(struct) lists fields in metadata order.
• Assigning to a resolved namespace member throws in strict mode.

Testing

• New integration-tests/tests/struct_values.rs and namespace_members.rs.
• All integration test binaries pass.
• Runtime unit tests show the same failures as main (11 known UI-thread/environment failures).
• cargo check passes for the napi-only build (--no-default-features --features napi_engine) and for the combined build.

Not in this PR

Windows.UI.ColorHelper.ToDisplayName(color) crashes with an access violation on main as well; small structs passed by value need a separate look.

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