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>

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "@colbymchenry/codegraph",
-  "version": "0.6.6",
+  "version": "0.6.8",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "@colbymchenry/codegraph",
-      "version": "0.6.6",
+      "version": "0.6.8",
       "hasInstallScript": true,
       "license": "MIT",
       "dependencies": {