docs: refresh for 0.2.2 layout, add Quick start, expose executables

[jwinarske]

pubspec.yaml

• Add issue_tracker pointing at the GitHub issues page.
• Declare an executables: block so dart pub global activate appstream_dart exposes bin/main.dart as appstream_dart and bin/query.dart as appstream_dart_query.

README.md

• Add a "Quick start" section as the first content block: a complete parse-and-query example using the canonical package:appstream_dart/appstream_dart.dart import, the pubspec.yaml dependency snippet, and the C++23 / CMake / SQLite prerequisites with a pointer to doc/ADVANCED_BUILD.md.
• Bump the Quick Facts status from v0.2.0 to v0.2.2.
• Project tree: tests/ -> native_tests/, docs/ -> doc/.
• Documentation table: all four doc/* links updated.

doc/

• ADVANCED_BUILD.md: 17 build*/tests/ paths -> build*/native_tests/, the lcov /tests/ exclusion -> /native_tests/, and the tests/test_real_appstream_data.cpp source path.
• ARCHITECTURE.md: tests/ -> native_tests/, docs/ -> doc/ in the build/test topology section.
• BUILD_SYSTEM.md: tests/CMakeLists.txt, tests/test_helpers.h, and build/tests/appstream_tests references migrated.
• COMPLETE_SUMMARY.md: directory tree (including the build/tests/ subtree), test-file enumeration, and source-files list all moved off tests/.
• RUNNING_TESTS.md: tests/*.cpp mention updated.
• TEST_SUMMARY.md: CMake module and test-file paths updated.

doc/CODE_AUDIT_REPORT.md is intentionally left untouched — it's a frozen historical audit, not instructional content.