feat(core): add Dart language support

[junddao]

Summary

Adds Dart to the tree-sitter pipeline so Flutter/Dart codebases get the same structural analysis (functions, classes, imports/exports) other languages already enjoy.

What's in this PR

• New dartConfig registered in the language registry (.dart ext, Dart-flavored concepts and file patterns).
• New DartExtractor mapping tree-sitter-dart's grammar to the shared StructuralAnalysis shape:
  • top-level function_signature / getter / setter → functions
  • class_definition / mixin_declaration / extension_declaration / enum_declaration → classes (methods + properties + enum constants)
  • library_import / library_export (wrapped in import_or_export) → imports, with the _-prefix Dart visibility convention for exports
• Adds tree-sitter-dart@^1.0.0 as a dependency. Its npm-published WASM ships an outdated dylink format that fails to load against web-tree-sitter 0.26.x, so this PR also bundles a freshly rebuilt WASM at understand-anything-plugin/packages/core/wasm-grammars/tree-sitter-dart.wasm and teaches TreeSitterPlugin to fall back to bundled grammars when the npm copy is missing or stale. The fallback mechanism is generic — any future grammar with a similar issue can drop a WASM into wasm-grammars/ and benefit from it.
• Tests: adds DartExtractor unit tests covering imports, top-level functions, classes/mixins/enums, and the _-prefix visibility rule; updates the language-registry built-in count from 40 → 41.

Notes

• extractCallGraph returns [] for Dart on purpose. tree-sitter-dart does not expose a single call_expression node — calls are encoded via chained identifier + argument_part selectors, which require nontrivial heuristics to disambiguate from member access. Semantic call relationships are still surfaced by the LLM-side analysis. Happy to follow up with a separate PR if maintainers want a structural call graph.
• Anonymous extensions (extension on Foo { ... }) get a synthetic name (_AnonymousExtension_<line>) so they don't collide; they are not added to exports.

Linked issue(s)

N/A

How I tested this

• pnpm lint
• pnpm --filter @understand-anything/core test — 678/678 passing (added 8 new DartExtractor unit tests)
• pnpm test — 201/201 passing
• Verified the WASM fallback path: with the stale npm WASM in place (which does not load), TreeSitterPlugin transparently uses the bundled wasm-grammars/tree-sitter-dart.wasm and Dart parsing succeeds.

Versioning

• Version bumped in all five manifests (2.7.6 → 2.8.0)