lowerComptimeCall stamped the caller's source onto fixed comptime `$`-params
so their substituted bare names resolve in the caller's visibility context,
but the variadic comptime pack branch (`..$args`) recorded the pack-arg slice
without stamping. Those nodes are later re-lowered via packArgNodeAt under the
defining-module pin, so a caller-owned helper in a formatted-arg position
(`std.print("{}", caller_fn())`) was checked against the metaprogram's module
and rejected as "not visible". Stamp every pack-arg node with the caller source,
mirroring the fixed-param treatment — completing Problem 1 for pack args.
Regression: examples/0739-modules-comptime-pack-arg-caller-context.sx
(two caller-owned s64 helpers in std.print pack positions; fail-before both
"not visible", pass-after prints "42 7"). No exemption flag, no silent default.
