refactor(ffi-linkage): Phase 9.2a — rename runtime-class TYPE names → Runtime* (Decision 5)

Mechanical, collision-free PascalCase renames (object-model axis, not linkage):
ForeignClassDecl→RuntimeClassDecl, ForeignMethodDecl→RuntimeMethodDecl,
ForeignClassMember→RuntimeClassMember, ForeignFieldDecl→RuntimeFieldDecl,
ForeignRuntime→RuntimeKind, ForeignClassPrefix→RuntimeClassPrefix. Snapshot-neutral;
suite green (646/444). Remaining 9.2: snake_case state (foreign_class_map,
current_foreign_class, foreign_path [coupled to .sx hooks], the foreign_class_decl
union variant) + the parse/lower/resolve fn names + ForeignClassDecl.is_foreign flag.

src/ir/program_index.test.zig

@@ -67,8 +67,8 @@ test "ProgramIndex declaration maps round-trip (A1.1b)" {
     try idx.module_const_map.put("AF_INET", .{ .value = &blk, .ty = .i32 });
     try std.testing.expect(idx.module_const_map.get("AF_INET").?.value == &blk);
 
-    // foreign_class_map: sx alias → ForeignClassDecl.
-    const fcd = ast.ForeignClassDecl{
+    // foreign_class_map: sx alias → RuntimeClassDecl.
+    const fcd = ast.RuntimeClassDecl{
         .name = "NSString",
         .foreign_path = "NSString",
         .runtime = .objc_class,