fix(generator2): dedupe transitive shared fragment imports

[knaeckeKami]

Summary

• dedupe transitive shared fragment imports by traversing document sources uniquely by URL
• memoize source loading by AssetId so shared imports are parsed once across recursive reads
• add generator and end-to-end regression coverage for shared transitive fragments

Testing

• ./.github/workflows/scripts/validate-formatting.sh
• dart test test/source_test.dart test/build_runner_smoke_test.dart test/end_to_end_mirror_test.dart
• dart test test/runtime_test.dart
• dart analyze

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