feat: Improve Java extraction, multi-symbol aggregation, and impact traversal

Java extraction:
- Handle Java method_invocation AST (receiver.method pattern)
- Support Java extends_interfaces and super_interfaces with type_list
- Create unresolved references for Java imports for cross-file resolution
- Extract interface inheritance via extractInheritance

MCP tools:
- Aggregate callers/callees/impact across ALL matching symbols (e.g. multiple
  overloads or same-named methods in different classes)
- New findAllSymbols() helper for multi-symbol lookup

Graph traversal:
- Impact analysis now traverses into container children (class → methods)
  so that callers of methods appear in the impact radius of their class

Other:
- Add deleteSpecificResolvedReferences() for precise cleanup after resolution
- Add 'instance-method' to resolvedBy union type
- Version bump to 0.6.8

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/resolution/types.ts

@@ -39,7 +39,7 @@ export interface ResolvedRef {
   /** Confidence score (0-1) */
   confidence: number;
   /** How it was resolved */
-  resolvedBy: 'exact-match' | 'import' | 'qualified-name' | 'framework' | 'fuzzy';
+  resolvedBy: 'exact-match' | 'import' | 'qualified-name' | 'framework' | 'fuzzy' | 'instance-method';
 }
 
 /**