docs(changelog): add Objective-C indexing entry under [Unreleased] (#429)

Covers #165: tree-sitter-objc extractor for .m / .mm / content-sniffed
.h, with full multi-part selectors, @protocol nodes, @property, message
expression call edges, extends/implements edges. Validated on
AFNetworking / RestKit / Texture. Disclosed limitations match the
README's 'Partial support' note (categories produce duplicate class
nodes per category file; .mm ObjC++ parses incompletely under the ObjC
grammar; mixed Swift/ObjC bridging out of scope, tracked separately).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>