feat(extraction): add Objective-C language support (#165)

Adds tree-sitter-objc extractor for `.m`/`.mm` files and `.h` files
that content-sniff as Objective-C (`@interface`/`@implementation`/`@protocol`/`@synthesize`).

Extraction covers:
- `@interface` / `@implementation` (deduplicated into a single class node)
- `@protocol` (as `protocol` nodes via new `interfaceKind` config)
- Methods with full multi-part selectors (`doThing:with:`, not just `doThing`),
  including `+`/`-` static distinction
- `@property` declarations
- Inheritance (`extends`) and protocol conformance (`implements`)
- C-style `function_definition` and `#import` (both `<system>` and `"local"` forms)
- Call edges from both `call_expression` and `message_expression`,
  with `self`/`super` skipped on qualified callee names

Two new generic hooks on `LanguageExtractor` (`resolveName`,
`extractPropertyName`) handle the cases where the default name walk
doesn't fit; usable by future languages with similar shape.

Import resolver tries `.h`, `.m`, `.mm` for `objc` imports.

Validated on AFNetworking (84 files, 100% file coverage), RestKit
(282 files, 99.6%), and Texture (926 files, 100%, heavy `.mm`
content) — multi-keyword selectors preserved up to 7 parts, no parse
failures on ObjC++.

Known limitations (disclosed in README):
- Categories produce duplicate class nodes (one per category file)
- Chained/nested message sends record only the innermost method
- `[Class alloc]` patterns don't emit `instantiates` edges
- `@protocol Foo <Bar>` refinement lists not yet wired to `implements`
- Heavy C++ in `.mm` files may parse incompletely under the ObjC grammar

src/extraction/tree-sitter-types.ts

@@ -120,6 +120,12 @@ export interface LanguageExtractor {
 
   // --- Existing hooks ---
 
+  /** Override symbol name extraction (e.g. ObjC multi-part selectors). */
+  resolveName?: (node: SyntaxNode, source: string) => string | undefined;
+
+  /** Extract property name when the generic name walk fails (e.g. ObjC @property). */
+  extractPropertyName?: (node: SyntaxNode, source: string) => string | null;
+
   /** Extract signature from node */
   getSignature?: (node: SyntaxNode, source: string) => string | undefined;
   /** Extract visibility from node */