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.

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