ffi M1.2 A.0: objc_defined_class_cache + scan-pass registration

Adds an insertion-ordered cache on Module for sx-defined Obj-C
classes — every '#objc_class("Cls") { ... }' declaration WITHOUT
'#foreign'. registerForeignClassDecl appends the entry alongside
its existing foreign_class_map insert; lookup helper available
via Module.lookupObjcDefinedClass.

  ObjcDefinedClassEntry { name, *const ast.ForeignClassDecl }

The pointer back into the AST lets later passes (M1.2 A.1+) walk
'members' for fields / methods / '#extends' / '#implements'
without duplicating that data on the entry. Insertion order
matters because class-pair init constructors (A.4) must register
parent classes before children — 'objc_allocateClassPair(super,
...)' resolves super by lookup.

Infrastructure only — no observable behavior change. The cache
is populated but not yet read; A.1+ start pulling from it. 170
example tests + zig build test green.

src/ir/lower.zig

@@ -9413,8 +9413,17 @@ pub const Lowering = struct {
     /// type (e.g. `*Activity`) is resolved via the existing struct
     /// fallback in `type_bridge.resolveTypeName` (which interns unknown
     /// named types as 0-field structs).
+    ///
+    /// sx-defined Obj-C classes (no `#foreign`, runtime == .objc_class)
+    /// also land in `module.objc_defined_class_cache` in declaration
+    /// order — that cache drives M1.2 class-synthesis emission (A.4+).
     fn registerForeignClassDecl(self: *Lowering, fcd: *const ast.ForeignClassDecl) void {
         self.foreign_class_map.put(fcd.name, fcd) catch {};
+        if (!fcd.is_foreign and fcd.runtime == .objc_class) {
+            if (self.module.lookupObjcDefinedClass(fcd.name) == null) {
+                self.module.appendObjcDefinedClass(fcd.name, fcd);
+            }
+        }
     }
 
     /// Lazily declare the `sx_jni_env_tl_get` / `sx_jni_env_tl_set`

src/ir/module.zig

@@ -2,6 +2,7 @@ const std = @import("std");
 const Allocator = std.mem.Allocator;
 const types = @import("types.zig");
 const inst = @import("inst.zig");
+const ast = @import("../ast.zig");
 
 const TypeId = types.TypeId;
 const TypeInfo = types.TypeInfo;
@@ -41,6 +42,15 @@ pub const Module = struct {
     /// `Cls.static_method(...)` against an `#objc_class` alias resolves
     /// the class object through this cache once per module.
     objc_class_cache: std.ArrayList(ObjcClassEntry),
+    /// sx-defined Obj-C classes — every `Cls :: #objc_class("Cls") { ... }`
+    /// declaration WITHOUT `#foreign`. Insertion-ordered so the
+    /// class-registration constructors (M1.2 A.4) emit in source order
+    /// — parent classes register before children, which matters because
+    /// `objc_allocateClassPair(super, ...)` resolves `super` by lookup.
+    /// Each entry holds a pointer back into the AST so later passes
+    /// (trampoline emission, +alloc/-dealloc synthesis) can re-walk
+    /// `members` for fields / methods / `#extends` / `#implements`.
+    objc_defined_class_cache: std.ArrayList(ObjcDefinedClassEntry),
     alloc: Allocator,
     /// True when this module's program imports `std.sx` (and therefore
     /// has the `Context` type). Set by lowering's Pass 0 pre-scan. Read
@@ -50,6 +60,10 @@ pub const Module = struct {
 
     pub const ObjcSelectorEntry = struct { sel: []const u8, slot: GlobalId };
     pub const ObjcClassEntry = struct { name: []const u8, slot: GlobalId };
+    /// Pointer back to the AST node lets later passes re-walk `members`
+    /// for fields / methods / `#extends` / `#implements` without
+    /// duplicating that data here.
+    pub const ObjcDefinedClassEntry = struct { name: []const u8, decl: *const ast.ForeignClassDecl };
 
     pub fn init(alloc: Allocator) Module {
         return .{
@@ -59,6 +73,7 @@ pub const Module = struct {
             .impl_table = ImplTable.init(alloc),
             .objc_selector_cache = std.ArrayList(ObjcSelectorEntry).empty,
             .objc_class_cache = std.ArrayList(ObjcClassEntry).empty,
+            .objc_defined_class_cache = std.ArrayList(ObjcDefinedClassEntry).empty,
             .alloc = alloc,
         };
     }
@@ -72,6 +87,7 @@ pub const Module = struct {
         self.impl_table.deinit();
         self.objc_selector_cache.deinit(self.alloc);
         self.objc_class_cache.deinit(self.alloc);
+        self.objc_defined_class_cache.deinit(self.alloc);
         self.types.deinit();
     }
 
@@ -101,6 +117,18 @@ pub const Module = struct {
         self.objc_class_cache.append(self.alloc, .{ .name = name, .slot = slot }) catch unreachable;
     }
 
+    /// Linear scan over sx-defined Obj-C classes.
+    pub fn lookupObjcDefinedClass(self: *const Module, name: []const u8) ?*const ast.ForeignClassDecl {
+        for (self.objc_defined_class_cache.items) |entry| {
+            if (std.mem.eql(u8, entry.name, name)) return entry.decl;
+        }
+        return null;
+    }
+
+    pub fn appendObjcDefinedClass(self: *Module, name: []const u8, decl: *const ast.ForeignClassDecl) void {
+        self.objc_defined_class_cache.append(self.alloc, .{ .name = name, .decl = decl }) catch unreachable;
+    }
+
     pub fn addFunction(self: *Module, func: Function) FuncId {
         const id = FuncId.fromIndex(@intCast(self.functions.items.len));
         self.functions.append(self.alloc, func) catch unreachable;