fix(lower): stamp caller source on variadic comptime pack args [stdlib B attempt-6]

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.

examples/0739-modules-comptime-pack-arg-caller-context.sx

@@ -0,0 +1,25 @@
+// Caller-owned helpers passed as VARIADIC comptime-pack args (`..$args`) to a
+// NAMESPACED imported metaprogram resolve in the CALLER's visibility context —
+// not the metaprogram's defining module (regression, issue 0106 follow-up).
+//
+// `std.print :: ($fmt: string, ..$args)` is authored in `std.sx`; the pack args
+// `caller_num()` / `caller_two()` are authored HERE in the caller. The body's
+// typed `args[i]` substitution (via packArgNodeAt) lowers each pack arg under
+// the metaprogram's defining-module pin, so without stamping the pack-arg nodes
+// with the caller's source, the bare names `caller_num` / `caller_two` were
+// wrongly checked against `std.sx` and rejected as "not visible". The fixed
+// comptime param (`$fmt`) already got this treatment; this extends it to every
+// node in the variadic pack. The metaprogram's OWN code (build_format / out)
+// still resolves in `std.sx`, so the defining-context pin stays intact.
+//
+// Two pack positions lock that EVERY pack arg is stamped, not just the first.
+// s64 values only — accepted by print at runtime today (no 0107/0108 coupling).
+std :: #import "modules/std.sx";
+
+caller_num :: () -> s64 { return 42; }
+caller_two :: () -> s64 { return 7; }
+
+main :: () -> s32 {
+    std.print("{} {}\n", caller_num(), caller_two());
+    return 0;
+}

examples/expected/0739-modules-comptime-pack-arg-caller-context.exit

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

examples/expected/0739-modules-comptime-pack-arg-caller-context.stderr

@@ -0,0 +1 @@
+

examples/expected/0739-modules-comptime-pack-arg-caller-context.stdout

@@ -0,0 +1 @@
+42 7