feat: add Objective-C language support

[aaaron7]

Summary

Add Objective-C language support via tree-sitter-objc grammar, enabling proper callers/callees relationship extraction for Objective-C codebases.

Changes

• src/types.ts — Add 'objc' to LANGUAGES union type
• src/extraction/grammars.ts — Register tree-sitter-objc.wasm grammar, .m/.mm file extensions, and display name
• src/extraction/languages/objc.ts (new) — Full ObjC extractor supporting:
  • message_expression (method calls) — creates call edges
  • class_interface, class_implementation, protocol_declaration — container nodes
  • method_definition / method_declaration — function nodes with signatures
  • extractBareCall — Critical fix: visitFunctionBody bypasses the main visitNode dispatch, which means message_expression nodes inside method bodies were never captured. extractBareCall handles this, taking call edges from ~735 to 33,000+ on a real codebase.
  • Method signatures built from parameter selectors (e.g. -/+ + selector parts + parameter labels)
• src/extraction/languages/index.ts — Import and register ObjC extractor

Testing

• npm run build passes cleanly with tsc --noEmit
• Tested on a real iOS codebase: call edges increased from 735 → 33,000+ after this fix

[colbymchenry]

Thanks for taking the time to put this together, @aaaron7 — and especially
for the work of running it on a real iOS codebase to validate the call-edge
extraction was firing at all.

Closing this in favor of #165, which was opened a week earlier and takes
the same approach with a few key differences that turn out to matter for
correctness on real Objective-C code:

• Full multi-part selectors. ObjC method symbols need every keyword
  (e.g. setWidth:height:, not just setWidth); otherwise distinct methods
  collide. Tree-sitter-objc emits each keyword as a separate identifier
  sibling — they have to be joined back together with : separators. This
  one is the load-bearing fix: without it, the high call-edge count looks
  impressive but reflects collisions, not coverage. (Same correction applies
  to message_expression callsites — each [recv a:1 b:2] emits multiple
  field-named method children, all of which need to participate in the
  selector.)
• Protocols extracted (@protocol Foo ... @end → protocol nodes).
• @property names found by walking `struct_declaration > struct_declarator

  pointer_declarator > identifier— there's nonamefield onproperty_declaration`.

• @implementation reuses the @interface's class node instead of
  creating a duplicate.
• .h content-sniffing so iOS headers are classified as objc rather
  than c.
• 7 tests + README + CHANGELOG + agent-eval corpus entries, validated on
  Masonry / FMDB / SDWebImage.