test(ir): lock call lowering with .ir snapshots + classification tests (A3.2 convergence step 1)

Test-first scaffolding before the CallPlan convergence — no call-code
change. Locks current call behavior so the later lowerCall rewrite is
guarded.

- .ir snapshots for representative call forms: 0031 (direct local-fn +
  dot-shorthand enum ctor), 0032 (UFCS/struct method), 0301 (closure/
  fn-pointer slot), 0400 (protocol dispatch + static-through-impl).
- New focused example 0044-basic-default-arg-expansion + .ir snapshot,
  pinning call-site default expansion (scale(5)->scale(ctx,5,2),
  label(1)->label(ctx,1,"v","!")). Foreign-class instance+static is
  already pinned by the existing FFI .ir set.
- Broaden calls.test.zig (scope-free classification): remaining reflection
  builtins, sqrt->f64, cast->resolved type arg, enum_literal->target_type.

1033 (#caller_location) was rejected as a snapshot: it embeds the absolute
source path as a length-typed string that normalize_ir can't reconcile;
default-arg coverage uses the path-free 0044 instead.

Gate green: zig build, zig build test, tests/run_examples.sh -> 357/0.

examples/0044-basic-default-arg-expansion.sx

@@ -0,0 +1,23 @@
+// Call-site default-argument expansion (`appendDefaultArgs` / `expandCallDefaults`
+// in lowerCall). A call that omits a trailing parameter with a default value
+// has the default expression spliced in at the call site; an explicit argument
+// overrides it. Two trailing defaults cover the "fill all remaining" path.
+// Path-free IR (literal defaults) so the `.ir` snapshot is location-stable.
+
+#import "modules/std.sx";
+
+scale :: (n: s32, factor: s32 = 2) -> s32 { n * factor }
+
+label :: (n: s32, prefix: string = "v", suffix: string = "!") -> s32 {
+    print("{}{}{}\n", prefix, n, suffix);
+    n
+}
+
+main :: () {
+    print("default: {}\n", scale(5));
+    print("explicit: {}\n", scale(5, 3));
+
+    _ = label(1);                 // both defaults filled
+    _ = label(2, "x");            // suffix default filled
+    _ = label(3, "y", "?");       // no defaults
+}

examples/expected/0044-basic-default-arg-expansion.exit

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

examples/expected/0044-basic-default-arg-expansion.stdout

@@ -0,0 +1,5 @@
+default: 10
+explicit: 15
+v1!
+x2!
+y3?