feat(core): scaffold DartExtractor + register in builtinExtractors

Empty extractor that satisfies the LanguageExtractor interface so the
plugin pipeline can load it. Real extraction logic lands in subsequent
TDD commits.

understand-anything-plugin/packages/core/src/plugins/extractors/dart-extractor.ts

@@ -0,0 +1,48 @@
+import type { StructuralAnalysis, CallGraphEntry } from "../../types.js";
+import type { LanguageExtractor, TreeSitterNode } from "./types.js";
+import { findChild, findChildren } from "./base-extractor.js";
+
+/**
+ * Whether a Dart name is exported.
+ *
+ * Dart's visibility rule is name-based and the INVERSE of Kotlin's: names
+ * starting with `_` are library-private, everything else is exported. There
+ * is no `public` / `private` keyword to inspect — only the leading character.
+ */
+function isExported(name: string): boolean {
+  return !name.startsWith("_");
+}
+
+/**
+ * Dart extractor for tree-sitter structural analysis + call graph.
+ *
+ * Approach (matching `KotlinExtractor` convention): mixin / extension / enum
+ * declarations are folded into `StructuralAnalysis.classes[]` because the
+ * shared schema does not have a first-class slot for them. Extension
+ * declarations without a name surface as `"on <TargetType>"` so they aren't
+ * silently dropped.
+ */
+export class DartExtractor implements LanguageExtractor {
+  readonly languageIds = ["dart"];
+
+  extractStructure(rootNode: TreeSitterNode): StructuralAnalysis {
+    const functions: StructuralAnalysis["functions"] = [];
+    const classes: StructuralAnalysis["classes"] = [];
+    const imports: StructuralAnalysis["imports"] = [];
+    const exports: StructuralAnalysis["exports"] = [];
+
+    // Implementation lands in subsequent tasks.
+    void rootNode;
+    void findChild;
+    void findChildren;
+    void isExported;
+
+    return { functions, classes, imports, exports };
+  }
+
+  extractCallGraph(rootNode: TreeSitterNode): CallGraphEntry[] {
+    // Implementation lands in a later task.
+    void rootNode;
+    return [];
+  }
+}

understand-anything-plugin/packages/core/src/plugins/extractors/index.ts

@@ -9,6 +9,7 @@ export { RubyExtractor } from "./ruby-extractor.js";
 export { PhpExtractor } from "./php-extractor.js";
 export { CppExtractor } from "./cpp-extractor.js";
 export { CSharpExtractor } from "./csharp-extractor.js";
+export { DartExtractor } from "./dart-extractor.js";
 export { KotlinExtractor } from "./kotlin-extractor.js";
 
 import type { LanguageExtractor } from "./types.js";
@@ -21,6 +22,7 @@ import { RubyExtractor } from "./ruby-extractor.js";
 import { PhpExtractor } from "./php-extractor.js";
 import { CppExtractor } from "./cpp-extractor.js";
 import { CSharpExtractor } from "./csharp-extractor.js";
+import { DartExtractor } from "./dart-extractor.js";
 import { KotlinExtractor } from "./kotlin-extractor.js";
 
 export const builtinExtractors: LanguageExtractor[] = [
@@ -33,5 +35,6 @@ export const builtinExtractors: LanguageExtractor[] = [
   new PhpExtractor(),
   new CppExtractor(),
   new CSharpExtractor(),
+  new DartExtractor(),
   new KotlinExtractor(),
 ];