atomics A.2b: real CAS emission (cmpxchg) + unit test (green)

emitAtomicCmpxchg: LLVMBuildAtomicCmpXchg (success/failure orderings,
singleThread=0) returns a {T, i1} pair; LLVMSetWeak for the weak variant. The
sx ?T result (null = SUCCESS) is built as { extractvalue 0 (actual value),
xor(extractvalue 1 (success), true) } -- has_value = NOT success. Integer-only
(recognizer guard), so never a pointer/niche optional.

examples/1702 green: successful CAS returns null (value updated), failing CAS
returns the actual value (unchanged), weak retry loop increments a counter
(100 -> 105). LLVM IR shows `cmpxchg ... acq_rel acquire` and `cmpxchg weak`.
Unit test `emit: atomic cmpxchg (strong + weak)` locks `cmpxchg` + the weak
marker. Suite green (718/0).

src/ir/emit_llvm.test.zig

@@ -280,6 +280,40 @@ test "emit: atomic rmw (add + signed/unsigned min)" {
     try std.testing.expect(std.mem.indexOf(u8, ir_str, "atomicrmw umin") != null); // unsigned u64
 }
 
+test "emit: atomic cmpxchg (strong + weak)" {
+    const alloc = std.testing.allocator;
+    var module = Module.init(alloc);
+    defer module.deinit();
+
+    var b = Builder.init(&module);
+
+    const opt_i64 = module.types.optionalOf(.i64); // ?i64 result type
+
+    _ = b.beginFunction(str(&module, "f"), &.{}, .i64);
+    const entry = b.appendBlock(str(&module, "entry"), &.{});
+    b.switchToBlock(entry);
+
+    const p = b.alloca(.i64);
+    const exp = b.constInt(1, .i64);
+    const des = b.constInt(2, .i64);
+    // strong CAS
+    _ = b.emit(.{ .atomic_cmpxchg = .{ .ptr = p, .cmp = exp, .new = des, .val_ty = .i64, .success_ordering = .acq_rel, .failure_ordering = .acquire, .weak = false } }, opt_i64);
+    // weak CAS
+    _ = b.emit(.{ .atomic_cmpxchg = .{ .ptr = p, .cmp = exp, .new = des, .val_ty = .i64, .success_ordering = .seq_cst, .failure_ordering = .seq_cst, .weak = true } }, opt_i64);
+    b.ret(exp, .i64);
+    b.finalize();
+
+    var emitter = LLVMEmitter.init(alloc, &module, "test_cas", .{});
+    defer emitter.deinit();
+    emitter.emit();
+
+    try std.testing.expect(emitter.verify());
+
+    const ir_str = emitter.dumpToString();
+    try std.testing.expect(std.mem.indexOf(u8, ir_str, "cmpxchg") != null);
+    try std.testing.expect(std.mem.indexOf(u8, ir_str, "cmpxchg weak") != null); // the weak marker
+}
+
 test "emit: comparison and branch" {
     const alloc = std.testing.allocator;
     var module = Module.init(alloc);