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.
223ec3d0b3