ffi M5.A.next.4A.bare.5: end-to-end smoke for bare $args + dynamic reflection

Step 4A final-slice's smoke test. Exercises the FULL surface
step 5's generic Into(Block) impl needs to operate:

  1. A pack-fn binds $args (whole pack as []Type).
  2. The body walks `list := $args` at INTERP time.
  3. Per position, calls `type_name(list[i])` — the dynamic
     form that emits `callBuiltin(.type_name, ...)` at lower
     time, dispatched at interp time to read the runtime
     Value.type_tag and return the concrete type name.

`examples/172-pack-builder-smoke.sx` exercises four call
shapes via #run:
  describe()                      → []
  describe(42)                    → [s64]
  describe(42, "hi")              → [s64, string]
  describe(true, 3.14, "x", 99)   → [bool, f64, string, s64]

Each call shape builds its own [N x Any] slice of .type_tag
values at lowering time, the interp walks the slice, and the
per-element type names come out kind-honestly.

212/212 example tests + zig build test green.

examples/172-pack-builder-smoke.sx

@@ -0,0 +1,48 @@
+// Variadic heterogeneous type packs — step 4A end-to-end
+// smoke. Exercises the FULL chain step 5's generic
+// `Into(Block)` impl needs:
+//
+//   1. A pack-fn that passes its bound `$args` to a builder
+//      via `#run` / `#insert` (here `#run` for simplicity).
+//   2. The builder receives a `[]Type` slice and walks it,
+//      calling `type_name(list[i])` per position.
+//   3. `type_name(list[i])` is the DYNAMIC form — the index_expr
+//      argument can't be statically resolved at lower time, so
+//      the lowering emits a `callBuiltin(.type_name, ...)` that
+//      the interp's arm handles by reading the runtime
+//      `Value.type_tag(TypeId)` and returning the per-position
+//      name.
+//
+// The smoke demonstrates that step 4's full surface — bare
+// `$args` (whole pack as []Type) + dynamic reflection
+// intrinsics + per-position type discovery at interp time —
+// hangs together end-to-end. This is the foundation step 5's
+// builder-driven generic Into(Block) impl rests on.
+
+#import "modules/std.sx";
+#import "modules/compiler.sx";
+
+// Generic "describe pack" builder. Receives the pack as
+// []Type, returns a joined string with each type's name.
+describe :: (..$args) -> string {
+    list := $args;
+    s := "[";
+    i : s64 = 0;
+    while i < list.len {
+        if i > 0 { s = concat(s, ", "); }
+        s = concat(s, type_name(list[i]));
+        i = i + 1;
+    }
+    s = concat(s, "]");
+    return s;
+}
+
+run_all :: () {
+    print("{}\n", describe());                        // []
+    print("{}\n", describe(42));                       // [s64]
+    print("{}\n", describe(42, "hi"));                 // [s64, string]
+    print("{}\n", describe(true, 3.14, "x", 99));       // [bool, f64, string, s64]
+}
+#run run_all();
+
+main :: () { print("rt\n"); }

tests/expected/172-pack-builder-smoke.exit

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

tests/expected/172-pack-builder-smoke.txt

@@ -0,0 +1,6 @@
+[]
+[s64]
+[s64, string]
+[bool, f64, string, s64]
+--- build done ---
+rt