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/imports.zig

@@ -463,7 +463,7 @@ pub const RawDeclRef = union(enum) {
     union_decl: *const ast.UnionDecl,
     error_set_decl: *const ast.ErrorSetDecl,
     protocol_decl: *const ast.ProtocolDecl,
-    foreign_class_decl: *const ast.ForeignClassDecl,
+    foreign_class_decl: *const ast.RuntimeClassDecl,
     namespace_decl: *const ast.NamespaceDecl,
 };
 
@@ -853,7 +853,7 @@ fn stampStructMethodSources(sd: ast.StructDecl, file_path: []const u8) void {
 
 /// Stamp the defining module path onto every bodied method of an sx-defined
 /// foreign class, so the method's sx body lowers in the class's own module.
-fn stampForeignClassMethodSources(fcd: ast.ForeignClassDecl, file_path: []const u8) void {
+fn stampForeignClassMethodSources(fcd: ast.RuntimeClassDecl, file_path: []const u8) void {
     for (fcd.members) |m| {
         if (m == .method) {
             if (m.method.body) |b| b.source_file = file_path;