ffi M5.A.next.2a.A: pack typed indexing — lock in Any-untyped miss

Step 2 of the variadic heterogeneous type packs feature: typed
runtime indexing (`args[$i]` at comptime-known `$i`). Today's
pack-fn body lowers `args[i]` through the `[]Any` slice path —
the static type returned is `Any`, so any downstream field
access / typed-coercion / further indexing fails the moment it
needs more than primitive auto-unboxing.

`examples/156-pack-typed-index.sx` pins the simplest visible
failure: `args[0].x` on a struct-typed call arg trips
"field 'x' not found on type 'Any'" at the field-access site
because AST-level type inference for `args[0]` returns Any.

Next commit teaches `lowerIndexExpr` (and `inferExprType` for
the same shape) to detect an index_expr whose base is a
pack-name binding from the enclosing comptime call AND whose
index is a comptime int literal — substitutes the i-th
call-site arg's lowered value directly, propagating the call
arg's concrete type through field access, typed assignments,
and further indexing. The `[]Any` slice path stays as the
runtime-indexed fallback for `args[i]` where `i` is not a
comptime constant.

195/195 example tests + `zig build test` green.

examples/156-pack-typed-index.sx

@@ -0,0 +1,32 @@
+// Variadic heterogeneous type packs — step 2: typed pack indexing.
+//
+// `args[$i]` (with `$i` a comptime-known integer) inside a pack-fn
+// body should resolve to the i-th call-site argument with its
+// CONCRETE type, not the boxed `Any` that today's `[]Any` slice
+// path yields. Without typed access, downstream operations on the
+// element (field access, typed coercion, passing to a typed slot)
+// either fail with "field 'X' not found on type 'Any'" or silently
+// box/unbox through Any.
+//
+// This file pins today's failure: `args[0].x` on a struct-typed
+// call arg trips "field 'x' not found on type 'Any'" because the
+// AST-level type inference for `args[0]` returns Any.
+//
+// Next commit teaches `lowerIndexExpr` to detect a pack-name base
+// with a comptime-int-literal index and substitute the i-th
+// call-site arg's lowered value directly — propagating the call
+// arg's real type through field access, typed assignments, and
+// further indexing.
+
+#import "modules/std.sx";
+
+Point :: struct { x: s64; y: s64; }
+
+get_x :: (..$args) -> s64 => args[0].x;
+
+main :: () -> s32 {
+    p := Point.{ x = 7, y = 9 };
+    n := get_x(p);
+    print("{}\n", n);
+    return 0;
+}

tests/expected/156-pack-typed-index.exit

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

tests/expected/156-pack-typed-index.txt

@@ -0,0 +1 @@
+/Users/agra/projects/sx/examples/156-pack-typed-index.sx:25:30: error: field 'x' not found on type 'Any'