ffi M5.A.next.2b.fu34.A: bare args + runtime index — lock in unresolved/LLVM errors

Lock-ins for follow-ups #3 (bare `args` reference) and #4
(`args[<runtime_int>]`) from step 2b. Both share the same root
cause: the pack-mono does not materialise an `[]Any` slice
value for the pack name, so any body that needs `args` as a
value at runtime fails.

`examples/162-pack-bare-args.sx` — pack-fn body forwards `args`
to a `[]Any`-typed helper. Today: "unresolved 'args' (in
... fn forward__pack_s64_string_f64)".

`examples/163-pack-runtime-index.sx` — pack-fn body indexes
`args[i]` with a runtime `i`. Today: LLVM verifier crash —
"GEP base pointer is not a vector or a vector of pointers" —
because `args` resolves to a junk Ref via the scope-lookup
fall-through, and the slice-indexing path emits a GEP off
that.

Next commit materialises an `[]Any` slice on demand inside the
mono: each pack param is boxed into Any, stored in a stack
[N x Any] array, and the slice {data_ptr, len} is bound to the
pack name. `args` then resolves as a runtime value the same way
the pre-2b inline path used to. `args[i]` runtime indexing goes
through the standard slice index path; element type is `Any`
(lossy on per-position types — inherent to runtime indexing
into a heterogeneous pack).

202/202 example tests + `zig build test` green.

examples/162-pack-bare-args.sx

@@ -0,0 +1,29 @@
+// Variadic heterogeneous type packs — follow-up #3 (bare `args`
+// reference). The pack-mono doesn't materialise a slice value
+// for the pack name itself, so any body that references `args`
+// without indexing (e.g. forwarding to a `[]Any`-typed helper,
+// or just computing `args.len` through a non-comptime path)
+// fails with "unresolved 'args'".
+//
+// Next commit materialises an `[]Any` slice on demand inside
+// the mono: each pack param is boxed into Any, stored in a
+// stack [N x Any] array, and the slice {data_ptr, len} is bound
+// to the pack name. `args` then resolves as a runtime value
+// like the pre-2b inline path did.
+
+#import "modules/std.sx";
+
+// Helper that takes a slice. Today the pack body can't pass
+// `args` here because `args` isn't in scope as a value.
+log_count :: (items: []Any) -> s64 {
+    return items.len;
+}
+
+forward :: (..$args) -> s64 {
+    return log_count(args);
+}
+
+main :: () -> s32 {
+    print("{}\n", forward(1, "hi", 2.5));
+    return 0;
+}

examples/163-pack-runtime-index.sx

@@ -0,0 +1,35 @@
+// Variadic heterogeneous type packs — follow-up #4 (runtime
+// pack indexing).
+//
+// `args[<literal>]` substitutes through `pack_arg_nodes` (step
+// 2a.B). But `args[<runtime_int>]` — a loop counter, an
+// expression — falls through to the standard slice-indexing
+// path, which fails because the pack-mono doesn't materialise
+// the `args` slice. Output today: "unresolved 'args'".
+//
+// Pairs with follow-up #3: the same `[]Any` slice
+// materialisation handles both `args` bare and `args[i]`
+// runtime. Element type becomes `Any` (lossy on per-position
+// types — that's the inherent trade-off of runtime indexing
+// into a heterogeneous pack).
+
+#import "modules/std.sx";
+
+count_anys :: (..$args) -> s64 {
+    total : s64 = 0;
+    i : s64 = 0;
+    while i < args.len {
+        // Runtime index — should resolve through the
+        // materialised slice once #3+#4 lands.
+        x : Any = args[i];
+        _ = x;
+        total = total + 1;
+        i = i + 1;
+    }
+    return total;
+}
+
+main :: () -> s32 {
+    print("{}\n", count_anys(10, "hi", 2.5, true));
+    return 0;
+}

tests/expected/162-pack-bare-args.exit

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

tests/expected/162-pack-bare-args.txt

@@ -0,0 +1 @@
+/Users/agra/projects/sx/examples/162-pack-bare-args.sx:23:22: error: unresolved 'args' (in /Users/agra/projects/sx/examples/162-pack-bare-args.sx fn forward__pack_s64_string_f64)

tests/expected/163-pack-runtime-index.exit

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

tests/expected/163-pack-runtime-index.txt

@@ -0,0 +1,5 @@
+LLVM verification failed: GEP base pointer is not a vector or a vector of pointers
+  %ig.ptr = getelementptr i64, i64 %ig.data, %load7
+Load operand must be a pointer.
+  %ig.val = load i64, i64 %ig.ptr, align 8
+