ffi M1.2 A.4b.iii: class_addMethod wires IMPs to the Obj-C runtime

For each instance method on a sx-defined '#objc_class', the
class-pair init constructor now:

  sel = sel_registerName("selector_string")
  imp = @__<Cls>_<method>_imp                  (M1.2 A.4b.ii)
  class_addMethod(cls, sel, imp, "<encoding>")

before objc_registerClassPair. The IMP trampoline (A.4b.ii)
already bridges C-ABI -> sx body. With registration in place,
'objc_msgSend(obj, sel_bump)' now routes to the trampoline,
which reads __sx_state ivar and forwards to '@<Cls>.<method>'.

To get selector + type-encoding strings out of lower.zig and
into emit_llvm, ObjcDefinedClassEntry gains a 'methods' slice:

  pub const ObjcDefinedMethodEntry = struct {
      sel: []const u8,       // mangled selector (M1.2 A.1's deriveObjcSelector)
      encoding: []const u8,  // type encoding (M1.2 A.1's objcTypeEncodingFromSignature)
      imp_name: []const u8,  // C-callconv trampoline symbol
  };

registerObjcDefinedClassMethods populates this when it declares
each method's body function; Module.setObjcDefinedClassMethods
attaches the slice to the cache entry by name. Static (class-
side) methods are skipped — A.4b only covers instance methods;
class-method hooks like '+layerClass' land in M2.1.

emit_llvm reads entry.methods and emits class_addMethod inside
the per-class init block, before objc_registerClassPair (the
runtime locks the method list at register time on some SDK
versions).

145-objc-class-method-dispatch.sx verifies end-to-end:
class_getMethodImplementation(SxFoo, sel_registerName("bump"))
returns non-null after main starts. Both niladic ('bump') and
single-arg ('add:') selectors checked.

Still gated (A.7): sx-side 'obj.bump()' calls. The dispatch
gate at lower.zig:4407 hasn't opened — A.5 (+alloc) and A.6
(-dealloc) need to land first so the integration test
ffi-objc-defined-class-01-instance.sx (full state round-trip)
can exercise the full lifecycle.

174 example tests pass (+1 from 145). zig build test green.

tests/expected/142-objc-class-method-lowering.ir

@@ -27,6 +27,8 @@
 @OBJC_IVAR_TYPE_ = private unnamed_addr constant [3 x i8] c"^v\00"
 @OBJC_CLASS_NAME_ = private unnamed_addr constant [9 x i8] c"NSObject\00"
 @OBJC_CLASS_NAME_.19 = private unnamed_addr constant [6 x i8] c"SxFoo\00"
+@OBJC_METH_VAR_NAME_ = private unnamed_addr constant [5 x i8] c"bump\00"
+@OBJC_METH_VAR_TYPE_ = private unnamed_addr constant [4 x i8] c"v@:\00"
 @llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 65535, ptr @__sx_objc_defined_class_init, ptr null }]
 
 ; Function Attrs: nounwind
@@ -804,6 +806,10 @@ declare ptr @objc_allocateClassPair(ptr, ptr, i64)
 
 declare i8 @class_addIvar(ptr, ptr, i64, i8, ptr)
 
+declare ptr @sel_registerName(ptr)
+
+declare i8 @class_addMethod(ptr, ptr, ptr, ptr)
+
 declare void @objc_registerClassPair(ptr)
 
 declare ptr @class_getInstanceVariable(ptr, ptr)
@@ -813,6 +819,8 @@ entry:
   %super_cls = call ptr @objc_getClass(ptr @OBJC_CLASS_NAME_)
   %cls = call ptr @objc_allocateClassPair(ptr %super_cls, ptr @OBJC_CLASS_NAME_.19, i64 0)
   %0 = call i8 @class_addIvar(ptr %cls, ptr @OBJC_IVAR_NAME_, i64 8, i8 3, ptr @OBJC_IVAR_TYPE_)
+  %sel = call ptr @sel_registerName(ptr @OBJC_METH_VAR_NAME_)
+  %1 = call i8 @class_addMethod(ptr %cls, ptr %sel, ptr @__SxFoo_bump_imp, ptr @OBJC_METH_VAR_TYPE_)
   call void @objc_registerClassPair(ptr %cls)
   %iv = call ptr @class_getInstanceVariable(ptr %cls, ptr @OBJC_IVAR_NAME_)
   store ptr %iv, ptr @__SxFoo_state_ivar, align 8

tests/expected/145-objc-class-method-dispatch.exit

@@ -0,0 +1 @@
+0

tests/expected/145-objc-class-method-dispatch.txt

@@ -0,0 +1 @@
+IMP: bump ok, add: ok