v0.3.0 of heimdall_test

Architecture rules for Dart source trees inspired by ArchUnit. Changelog excerpt: - Reworked the package around analyzer AST nodes as the primary source of rule information. - Expanded the fluent API for files, classes, members, constructors, code units, dependencies, layers, slices, plugins, and freezing. - Added broader built-in architecture checks for code hygiene, dependency policies, public class/file conventions, dynamic signatures, barrels, layers, and feature slices. - Improved importer behavior, package URI resolution, parse diagnostics, path matching, and dependency[...]