lang F1 2.6: pack-index edge cases (runtime-index error, comptime OOB)

Per locked Decision 1 a pack is comptime-only with no runtime value, so xs[i]
is valid only for a comptime index. lowerIndexExpr now emits a clear error
("pack <p> must be indexed by a compile-time constant ...") for a runtime
index, instead of the confusing "unresolved <p>" the slice-index fall-through
produced. diagPackIndexOOB switched from int-literal-only to comptimeIndexOf so
an inline-for cursor that goes out of bounds is also caught.

Repurposed examples/163-pack-runtime-index.sx (was aspirational: expected
runtime indexing to materialise a []Any slice and print 4, contradicting
Decision 1) into the runtime-index error test. Comptime + OOB cases already
covered by examples/199/200/161. 236 examples + unit green.

examples/163-pack-runtime-index.sx

@@ -1,17 +1,13 @@
-// Variadic heterogeneous type packs — follow-up #4 (runtime
-// pack indexing).
+// Variadic heterogeneous type packs — Step 2.6: indexing a pack with a
+// RUNTIME index is a compile error.
 //
-// `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).
+// Per locked Decision 1, a pack is comptime-only and has NO runtime
+// representation — so `args[i]` is valid only when `i` is a compile-time
+// constant (a literal, or an `inline for` cursor). A runtime index (here a
+// `while`-loop counter) must produce a clear diagnostic, not the confusing
+// "unresolved 'args'" the slice-index fall-through used to give. To walk a
+// pack, use `inline for 0..args.len (i) { ... }`, which unrolls so each
+// `args[i]` is a comptime index.
 
 #import "modules/std.sx";
 
@@ -19,9 +15,7 @@ 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 : Any = args[i];   // ERROR: runtime index into a comptime-only pack
         _ = x;
         total = total + 1;
         i = i + 1;