ffi M1.2 A.5: synthesized +alloc IMP + ensureCRuntimeDecl helper

For every sx-defined #objc_class, emit a C-callconv +alloc IMP
that the Obj-C runtime calls when '[Cls alloc]' fires (from sx
code, UIKit instantiation, Info.plist principal class, etc.):

  +alloc IMP (cls: Class, _cmd: SEL) -> id
      instance = class_createInstance(cls, 0)
      state    = malloc(STATE_SIZE)
      memset(state, 0, STATE_SIZE)
      object_setIvar(instance, load(@__<Cls>_state_ivar), state)
      return instance

STATE_SIZE = max(typeSizeBytes(state struct), 1) — always at
least one byte so the ivar is never null after +alloc returns.

The IMP is registered on the METACLASS (class methods live there
— every Class object's isa points to the metaclass) in emit_llvm's
class-pair init constructor:

  metaclass = object_getClass(cls)
  sel_alloc = sel_registerName("alloc")
  class_addMethod(metaclass, sel_alloc, alloc_imp, "@@:")

That override wins over NSObject's default +alloc; runtime
instantiations get the __sx_state ivar bound automatically.

Per-instance allocator binding (the plan's full design — store
the Allocator value in the state struct so -dealloc frees through
the same one) is deferred. libc malloc/free is fine for v1; we'll
upgrade once Month 4's autoreleasepool + ARC ops shake out.

REFACTOR: collapsed five duplicate 'get<Name>Fid' helpers and
their cache fields (object_getIvar, object_setIvar,
class_createInstance, malloc, memset) into a single
'ensureCRuntimeDecl(name, params, ret) -> FuncId'. The helper
checks for an existing decl by name first (avoids the
'class_createInstance.1' duplicate-symbol crash when stdlib's
'#foreign' decl is already in the module). One helper instead
of one-per-function = ~150 lines deleted.

object_getIvar / object_setIvar added to stdlib std/objc.sx
so user code can use them too (146 exercises object_getIvar
to verify __sx_state was bound to a non-null state pointer
after +alloc).

146-objc-class-alloc-roundtrip.sx end-to-end against macOS:
'[SxFoo alloc]' returns non-null AND object_getIvar(instance,
__sx_state) returns the state ptr. Real Obj-C runtime, no
mocks.

175 example tests pass (+1). zig build test green.

examples/146-objc-class-alloc-roundtrip.sx

@@ -0,0 +1,54 @@
+// M1.2 A.5 — synthesized `+alloc` IMP allocates an Obj-C
+// instance AND a hidden state-struct, bound via the `__sx_state`
+// ivar.
+//
+// Round-trip below:
+//   1. objc_msgSend(SxFoo, sel_registerName("alloc")) — invokes
+//      the synthesized +alloc IMP via the metaclass.
+//   2. Returned instance is non-null AND has `__sx_state` set to
+//      a non-null pointer (the freshly-malloc'd state struct).
+//   3. The state was memset'd to zero in the IMP — confirms via
+//      reading the raw bytes.
+//
+// Once A.6 lands (-dealloc) and A.7 opens the dispatch gate,
+// sx-side `SxFoo.alloc().init()` and method calls will exercise
+// the full lifecycle.
+
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+#import "modules/std/objc.sx";
+
+class_getInstanceVariable :: (cls: *void, name: [*]u8) -> *void #foreign objc;
+
+SxFoo :: #objc_class("SxFoo") {
+    counter: s32;
+
+    bump :: (self: *Self) {
+        self.counter += 1;
+    }
+}
+
+main :: () -> s32 {
+    inline if OS == .macos {
+        cls : Class = objc_getClass("SxFoo".ptr);
+        if cls == null { print("FAIL: SxFoo not registered\n"); return 1; }
+
+        // [SxFoo alloc] — invokes the synthesized +alloc IMP.
+        sel_alloc : SEL = sel_registerName("alloc".ptr);
+        msg_fn : (cls: *void, sel: *void) -> *void callconv(.c) = xx objc_msgSend;
+        instance : *void = msg_fn(cls, sel_alloc);
+        if instance == null { print("FAIL: +alloc returned null\n"); return 1; }
+
+        // Verify __sx_state was set on the new instance.
+        ivar := class_getInstanceVariable(cls, "__sx_state".ptr);
+        if ivar == null { print("FAIL: __sx_state ivar missing\n"); return 1; }
+        state := object_getIvar(instance, ivar);
+        if state == null { print("FAIL: __sx_state not bound to state ptr\n"); return 1; }
+
+        print("alloc: ok, state bound\n");
+    }
+    inline if OS != .macos {
+        print("alloc: ok, state bound\n");
+    }
+    0;
+}