v0.21.0

This release lets native and Wasm bindings use objects with private const
constructors as defaults, including defaults in value-class input properties.

📦 Keep private Dart defaults

You no longer need to expose a public constructor or constant solely to make
these defaults available to JavaScript:

@NAPI()
class Point {
  const Point._(this.x);
  final int x;
}

const _origin = Point._(7);

@NAPI()
int coordinate({Point point = _origin}) => point.x;

@NAPI(classPolicy: ClassPolicy.value)
class Selection {
  Selection({this.point = _origin});
  final Point point;
}

@NAPI()
int selectedCoordinate(Selection selection) => selection.point.x;

coordinate();                          // 7
coordinate({point: undefined});        // 7
selectedCoordinate({});                // 7
selectedCoordinate({point: undefined}); // 7

The original Dart constant supplies the default. This works for named parameters
and the last optional positional parameter of exported functions, static methods,
reference-class constructors and value-class input constructors.

Instance methods still require a public constructor or const binding, preserving
the selected declaration's defaults when a subclass overrides the method. Earlier
optional positional parameters have the same restriction. Function values, type
literals and symbols inside inaccessible defaults remain unsupported.

Argument defaults

🔒 Retain resources inside private defaults

A private default object can now contain resources when each stored resource has
a selected resource view and an accessible const expression or public stored-field
path. The enclosing object itself can keep its constructor private.

These defaults receive the same operation leases as explicit inputs, including
Future and Stream calls and value-class construction. Resources are retained
before later constructors, getters or callbacks run; failures release accepted
uses.

A const default is shared across calls. Once its resource is closing or closed,
omitted calls fail; supply another open resource explicitly. Resource leaves
without an accessible expression remain unsupported.

Resource defaults

Regenerate bindings to use the new defaults. Both targets require Dart 3.13.5;
native packages support macOS arm64 with bridge ABI 15.

Full changelog