fix(dart): Widen preact_signals dependency constraint in a2ui_core to support ^7.0.0

Description

Currently, dart/a2ui_core/pubspec.yaml pins preact_signals to:

dependencies:
  preact_signals: ^1.9.4

When integrating a2ui_core with modern signal-based architectures and Flutter packages that depend on preact_signals: ^7.0.0 (such as signals_core 7.x or bloc_signals), Dart pub version solving fails:

Because bloc_signals >=0.2.4 depends on preact_signals ^7.0.0 and a2ui_core >=0.0.1-dev002 depends on preact_signals ^1.9.4, bloc_signals >=0.2.4 is incompatible with a2ui_core >=0.0.1-dev002.

Proposed Solution

Widen the dependency constraint in dart/a2ui_core/pubspec.yaml to:

dependencies:
  preact_signals: ">=1.9.4 <8.0.0"
  # or: ^1.9.4 || ^7.0.0

The reactive signal primitives used by a2ui_core (signal, computed, batch) maintain full API compatibility across 1.9.x and 7.x.

Would the maintainers be open to a pull request with this change?