ffi M5.A.next.4A.bare.4.A: dynamic type_name(args[i]) — expected-failing lock-in

Step 4A final follow-up's lock-in. `type_name(<arg>)` where
<arg> is NOT a statically resolvable type expression (e.g.
`list[i]` indexing into a `$args`-derived `[]Type` slice)
silently folds to "s64" today because `resolveTypeArg`'s
index_expr fall-through returns `.s64` (the catch-all `else =>
.s64` at the bottom of the switch).

This is exactly the kind of silent unimplemented arm the
project's REJECTED PATTERNS section forbids — the user gets
"s64" for every element of an arbitrary pack, not the per-
position concrete type they expect.

`examples/171-pack-dynamic-type-name.sx` exercises a builder-
shaped fn: walks `$args` via runtime indexing, calls
`type_name(list[i])` per position, concatenates the results.
For `walk(42, "hi")` the expected output is "s64string".
Today's output is "s64s64" — the silent fold strikes twice.

Cadence shape 2: expected output is the WORKING shape; today's
diff fails. Next commit teaches `tryLowerReflectionCall` to
detect "arg not statically resolvable" and emit a builtin_call
to `.type_name` so the interp's runtime arm (wired in commit
9600ba5, M5.A.next.4.1) handles the dynamic case.

210/210 + 1 expected-failing = 211 total. zig build test green.

examples/171-pack-dynamic-type-name.sx

@@ -0,0 +1,34 @@
+// Variadic heterogeneous type packs — step 4A final-slice
+// follow-up. `type_name(<dynamic-arg>)` where the argument is
+// NOT a static type expression (e.g. `list[i]` indexing into
+// a `$args`-derived `[]Type` slice) silently folds to "s64"
+// today because `resolveTypeArg`'s index_expr fall-through
+// returns `.s64`. That's exactly the kind of silent unimplemented
+// arm the CLAUDE.md REJECTED PATTERNS section forbids.
+//
+// Next commit teaches `tryLowerReflectionCall` to detect "arg
+// not statically resolvable" and emit a `builtin_call` to the
+// new `.type_name` builtin. The interp's arm (already wired in
+// commit 9600ba5) reads the runtime `.type_tag` Value and
+// returns the per-position concrete type name.
+//
+// Expected output after fix:
+//   s64string
+
+#import "modules/std.sx";
+
+walk :: (..$args) -> string {
+    list := $args;
+    s := "";
+    i : s64 = 0;
+    while i < list.len {
+        s = concat(s, type_name(list[i]));
+        i = i + 1;
+    }
+    return s;
+}
+
+main :: () -> s32 {
+    print("{}\n", walk(42, "hi"));
+    return 0;
+}

tests/expected/171-pack-dynamic-type-name.exit

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

tests/expected/171-pack-dynamic-type-name.txt

@@ -0,0 +1 @@
+s64string