fix: struct constructors on napi; boxed struct values on both engines - #28

Stacked on #26 (base perf/struct-values-namespaces). GitHub retargets it to main when #26 merges.

Summary

Three struct gaps, fixed on both engines where they applied.

1. Struct constructors on napi

napi namespaces returned undefined for struct types (Windows.Foundation.Rect). They now resolve to a constructor:

new Rect({ X: 1, Y: 2, Width: 3, Height: 4 }) // named
new Rect(5, 6, 7, 8)                         // positional
new Rect()                                   // all zero
new Rect({ X: 1 })                           // missing fields zero
new Duration({ TimeSpan: { Duration: 5 }, Type: 1 }) // nested

The result is the same plain object a struct return gives. It's built by serializing through the struct's byte layout and reading back, so values match exactly what native receives.

2. Boxed structs (IReference<Point> etc.)

An IPropertyValue holding a Point, Size, Rect, TimeSpan or DateTime used to come back as an empty wrapper: {} on napi, { handle } on classic. It now unboxes to a struct value, e.g. PropertyValue.CreatePoint({X:1.5,Y:2.5}) gives { X: 1.5, Y: 2.5 }, and so does PropertySet.Lookup for a stored Point.

To keep the "create a value and store it in settings" pattern working, a plain object passed to an Object parameter is now boxed by its shape. {X,Y} becomes a Point, {Width,Height} a Size, {X,Y,Width,Height} a Rect, {Duration} a TimeSpan, and {UniversalTime} a DateTime. These are the only struct types IPropertyValue supports, and their field names don't overlap. So propertySet.Insert('p', { X: 1, Y: 2 }) now works directly.

3. Serializer fixes (both engines)

• Missing struct fields were sent as NaN for float fields (e.g. region.Padding = { Before: 1 }). They're now 0.
• napi String struct fields are written instead of zeroed (classic got this in perf: plain struct values and cached namespace lookups (classic) #26).

Also checked

• By-value small structs are fine on both engines. new Windows.UI.Input.Inking.InkPoint({X:3,Y:4}, 0.5).Position round-trips (Point, 8 bytes, passed in a register).
• Test fix pushed to perf: plain struct values and cached namespace lookups (classic) #26. Its round-trip test used TimedTextStyle.Padding, which doesn't exist (that class has no Padding), so it only round-tripped a JS property. It now uses TimedTextRegion.Padding and asserts no own property was created.

Testing

• Classic: new integration-tests/tests/boxed_structs.rs; all 18 integration test targets pass; runtime unit tests show the same 11 known failures as main.
• napi: npm test passes 417/417, including 11 new checks in struct-test.js.