refactor(ffi-linkage): Phase 6.4 — migrate ffi/ #foreign→extern

Pure source rename across objc/objc_block/raylib/sdl3/wasm (~51 sites): fn-decl
markers (bare / 'objc' LIB ref) → 'extern …', and objc.sx's 2 import runtime
classes '#foreign #objc_class("X") {' → '#objc_class("X") extern {'. No bare
defined classes. Behavior-preserving. objc + objc_block validated directly by the
50 marked 13xx corpus examples (incl. import classes 1300/1301 + defined classes
1339/1349); raylib/ffi-sdl3/wasm (no marked importers on host) verified by
byte-identical 'sx ir' probes pre/post. Empty snapshot diff; suite green (647
corpus / 444 unit, 0 failed).

library/modules/ffi/objc_block.sx

@@ -49,7 +49,7 @@ BlockDescriptor :: struct {
 
 // libSystem isa pointer for stack-allocated blocks. Resolved at link time
 // (auto-linked on every Apple target via libSystem).
-_NSConcreteStackBlock : *void #foreign;
+_NSConcreteStackBlock : *void extern;
 
 // Shared descriptor for the 48-byte sx-block layout. All Into impls below
 // point their `descriptor` field at this.