ffi 3.0 (xfail): add inst.method(args) DSL regression tests + correct checkpoint

The previous FFI checkpoint claimed Phase 3 step 3.0 ("`inst.method(args)`
on #objc_class receivers") had landed. It hadn't — `lowerForeignMethodCall`
in lower.zig:4353 still bails for any non-JNI runtime with the generic
"method calls on '{runtime}' runtime not yet supported (Phase 3/4)"
diagnostic, no commit introduced an Obj-C DSL dispatch path, and the
planned regression files weren't on disk.

This commit is the xfail half of the proper cadence (test-add then
make-green in separate commits):

- examples/ffi-objc-dsl-01-niladic.sx — `length()` → selector "length".
- examples/ffi-objc-dsl-02-one-arg.sx — `addObject(o)` → "addObject:".
- examples/ffi-objc-dsl-03-multi-keyword.sx — `combine_and(a, b)` →
  "combine:and:" (sx name split on `_`, each piece becomes a keyword
  with a trailing `:`).
- examples/ffi-objc-dsl-04-mismatch.sx — `something_extra(x)` —
  keyword count (2) ≠ arity (1); must diagnose at the call site.

Each test follows the same pattern as `ffi-objc-call-08-multi-keyword.sx`:
synthesize a class at runtime via `objc_allocateClassPair` /
`class_addMethod`, declare the sx-side `#objc_class` against the same
name, then invoke the DSL form. Skips with a "(not macos)" line on
non-macOS hosts. Snapshots currently lock in the bail diagnostic with
exit=1; the next commit implements the dispatch and the snapshots
flip to the working output (and exit=0).

Checkpoint corrected to flag the prior false claim and reposition 3.0
back at the top of the open list.

examples/ffi-objc-dsl-01-niladic.sx

@@ -0,0 +1,43 @@
+// Phase 3 step 3.0 (PLAN-FFI.md): `inst.method(args)` on an
+// `#objc_class`-typed receiver lowers to `objc_msg_send` with a derived
+// selector. This test covers the niladic shape: the sx-side method name
+// is used verbatim as the selector (`length` → `length`).
+//
+// Test pattern mirrors `ffi-objc-call-08-multi-keyword.sx`: synthesize a
+// fresh class at runtime via `objc_allocateClassPair` + `class_addMethod`,
+// declare the sx-side `#objc_class` against the same name, then invoke
+// the DSL form. On macOS this exercises the real Obj-C runtime; on
+// other platforms the test skips.
+//
+// Pre-3.0: the dispatch bails at lower.zig with "method calls on
+// 'objc_class' runtime not yet supported (Phase 3/4)". Snapshot captures
+// that diagnostic.
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+#import "modules/std/objc.sx";
+
+SxProbeNiladic :: #foreign #objc_class("SxProbeNiladic") {
+    length :: (self: *Self) -> s32;
+}
+
+length_imp :: (self: *void, _cmd: *void) -> s32 callconv(.c) {
+    42;
+}
+
+main :: () -> s32 {
+    inline if OS == .macos {
+        ns_object := objc_getClass("NSObject".ptr);
+        cls := objc_allocateClassPair(ns_object, "SxProbeNiladic".ptr, 0);
+        sel := sel_registerName("length".ptr);
+        class_addMethod(cls, sel, xx length_imp, "i@:".ptr);
+        objc_registerClassPair(cls);
+
+        inst : *SxProbeNiladic = xx class_createInstance(cls, 0);
+        n := inst.length();
+        print("length = {}\n", n);
+    }
+    inline if OS != .macos {
+        print("skipped (not macos)\n");
+    }
+    0;
+}

examples/ffi-objc-dsl-02-one-arg.sx

@@ -0,0 +1,35 @@
+// Phase 3 step 3.0: one-arg selector mangling. `addObject(o)` derives
+// `addObject:` — the sx method name becomes the keyword, a single
+// trailing `:` for the single arg. Selector arity (count of `:`) must
+// equal sx-side arity excluding self.
+//
+// Pre-3.0: bails at lower.zig with the Phase 3/4 diagnostic.
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+#import "modules/std/objc.sx";
+
+SxProbeOneArg :: #foreign #objc_class("SxProbeOneArg") {
+    addObject :: (self: *Self, val: s32) -> s32;
+}
+
+addObject_imp :: (self: *void, _cmd: *void, val: s32) -> s32 callconv(.c) {
+    val * 2;
+}
+
+main :: () -> s32 {
+    inline if OS == .macos {
+        ns_object := objc_getClass("NSObject".ptr);
+        cls := objc_allocateClassPair(ns_object, "SxProbeOneArg".ptr, 0);
+        sel := sel_registerName("addObject:".ptr);
+        class_addMethod(cls, sel, xx addObject_imp, "i@:i".ptr);
+        objc_registerClassPair(cls);
+
+        inst : *SxProbeOneArg = xx class_createInstance(cls, 0);
+        n := inst.addObject(21);
+        print("addObject(21) = {}\n", n);
+    }
+    inline if OS != .macos {
+        print("skipped (not macos)\n");
+    }
+    0;
+}

examples/ffi-objc-dsl-03-multi-keyword.sx

@@ -0,0 +1,34 @@
+// Phase 3 step 3.0: multi-keyword selector mangling. The sx method
+// name is split on `_`; each piece becomes a keyword with a trailing
+// `:`. `combine_and(a, b)` → `combine:and:` — two keywords, two args.
+//
+// Pre-3.0: bails at lower.zig with the Phase 3/4 diagnostic.
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+#import "modules/std/objc.sx";
+
+SxProbeMultiKeyword :: #foreign #objc_class("SxProbeMultiKeyword") {
+    combine_and :: (self: *Self, a: s32, b: s32) -> s32;
+}
+
+combine_imp :: (self: *void, _cmd: *void, a: s32, b: s32) -> s32 callconv(.c) {
+    a * 100 + b;
+}
+
+main :: () -> s32 {
+    inline if OS == .macos {
+        ns_object := objc_getClass("NSObject".ptr);
+        cls := objc_allocateClassPair(ns_object, "SxProbeMultiKeyword".ptr, 0);
+        sel := sel_registerName("combine:and:".ptr);
+        class_addMethod(cls, sel, xx combine_imp, "i@:ii".ptr);
+        objc_registerClassPair(cls);
+
+        inst : *SxProbeMultiKeyword = xx class_createInstance(cls, 0);
+        n := inst.combine_and(7, 42);
+        print("combine_and(7, 42) = {}\n", n);
+    }
+    inline if OS != .macos {
+        print("skipped (not macos)\n");
+    }
+    0;
+}

examples/ffi-objc-dsl-04-mismatch.sx

@@ -0,0 +1,25 @@
+// Phase 3 step 3.0: keyword count must equal call-site arity (excluding
+// self). `something_extra(x)` — name split gives ["something", "extra"]
+// = 2 keywords; arity = 1. Compiler must diagnose at the call site.
+//
+// Pre-3.0: bails at lower.zig with the generic Phase 3/4 diagnostic
+// (which subsumes this case). Once 3.0 lands, the diagnostic becomes a
+// specific "keyword count mismatch" message.
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+
+SxProbeMismatch :: #foreign #objc_class("SxProbeMismatch") {
+    something_extra :: (self: *Self, x: s32) -> s32;
+}
+
+main :: () -> s32 {
+    inline if OS == .macos {
+        inst : *SxProbeMismatch = null;
+        n := inst.something_extra(7);
+        print("n = {}\n", n);
+    }
+    inline if OS != .macos {
+        print("skipped (not macos)\n");
+    }
+    0;
+}