fix(objc): resolve chained message-send calls [[Foo create] doIt] (#750) (#786)

Ports the #645/#608 chained-receiver mechanism to Objective-C. A message send
whose receiver is itself a message send — `[[Foo create] doIt]` — used to drop
the receiver, so `doIt` name-matched a same-named method on an unrelated class
(commonly a test helper's `init` or an Apple-SDK method).

- objc.ts: getReturnType reads the method's `method_type`, SKIPPING nullability /
  ARC qualifiers (`nonnull instancetype` must yield instancetype, not `nonnull`).
- tree-sitter.ts: the message_expression branch now re-encodes a chained send
  `[[Foo create] doIt]` as `Foo.create().doIt` when the inner receiver is a
  capitalized class and the outer selector is unary.
- name-matcher.ts: `objc` joins the dotted-chain gate + CHAIN_LANGUAGES. A
  class-message factory returns an instance of the RECEIVER class by convention
  (`instancetype`), so when the factory's own return type isn't recoverable
  (`alloc`/`new`/`shared…` return instancetype, or aren't user nodes), the
  receiver's type is the class itself — this resolves the ubiquitous
  `[[X alloc] init]` and singleton chains. resolveMethodOnType validates against
  the class and its supertypes, so a wrong inference yields no edge.

Validation: 4 synthetic tests (factory+decoy, superclass conformance, absent-method
safety, the nonnull-instancetype singleton). Real-repo A/B on SDWebImage (208 files):
+35 / -75 — all corrections (the -75 are wrong `init` mis-matches to a test helper /
wrong class, retargeted to the right class's init in the +35, plus 2 Apple-SDK chains
on unindexed classes). db stable, no node explosion. EXTRACTION_VERSION 14->15.
Full suite green.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/extraction/extraction-version.ts

@@ -21,4 +21,4 @@
  * turns the re-index hint into noise — keep it honest (see CLAUDE.md, "Honesty
  * in the product is load-bearing").
  */
-export const EXTRACTION_VERSION = 14;
+export const EXTRACTION_VERSION = 15;