refactor(ffi-linkage): Phase 9.2c — rename extern-ref validators → Extern (linkage)

checkForeignRefs→checkExternRefs, validateForeignRefs→validateExternRefs,
collectForeignRefTargets→collectExternRefTargets — these police 'extern LIB' library
references (linkage axis), so Extern not Runtime. Snapshot-neutral; suite green.

src/core.zig

@@ -302,7 +302,7 @@ pub const Compilation = struct {
         // Every `#foreign <ref>` must name a #library constant or a named
         // `#import c` unit — a typo'd ref otherwise resolves silently
         // through whatever image happens to carry the symbol.
-        try c_import.validateForeignRefs(self.allocator, root, &self.diagnostics);
+        try c_import.validateExternRefs(self.allocator, root, &self.diagnostics);
         if (self.diagnostics.hasErrors()) return error.CompileError;
 
         var module = ir.Module.init(self.allocator);