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-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;
+}