fix(lower): resolve substituted caller comptime $-args in caller context [stdlib B attempt-5]

attempt-3 pinned current_source_file to the metaprogram's defining module
across the whole body lowering (lowerComptimeCall / monomorphizePackFn). That
pin also covered caller-provided comptime $-arg nodes spliced into the body by
substituteComptimeNodes — but those are CALLER-authored and must resolve in the
caller's visibility context, not the callee's. Result: a caller-owned helper
passed to an imported metaprogram errored "'<name>' is not visible".

Fix: stamp each comptime $-arg node with the caller's source_file at the cpn
build site (stampCallerSource, in lowerComptimeCall + monomorphizePackFn);
lowerExpr switches current_source_file to a node's source_file when present, so
the substituted subtree resolves against the caller while the surrounding callee
code keeps the defining-module pin. No exemption / fall-open.

Regression: examples/0738-modules-comptime-arg-caller-context.sx — a caller-owned
helper passed as a comptime-ONLY $-arg through a namespaced import. Fail-before
(attempt-3 binary): "'caller_name' is not visible". Pass-after: prints
"hello world", exit 0. Comptime-only, so it does not exercise issue 0107.

0106 RESOLVED banner extended (point 3: body=defining context, substituted
$-args=caller context). run_examples 473 -> 474; zig build test 412/412.

examples/0738-modules-comptime-arg-caller-context.sx

@@ -0,0 +1,24 @@
+// A caller-owned helper passed as a comptime-ONLY `$`-arg to a NAMESPACED
+// imported metaprogram resolves in the CALLER's visibility context — not the
+// metaprogram's defining module (regression, issue 0106 follow-up).
+//
+// `emit` is reachable only as `m.emit`; the comptime arg `caller_name()` is
+// authored HERE in the caller. When `emit` splices that arg into its `#insert`
+// body and lowers it, the bare name `caller_name` must stay visible in the
+// caller's context. Before the fix, the body's defining-module pin also covered
+// the substituted caller arg, so `caller_name` was wrongly checked against
+// `emit.sx` and rejected as "not visible". The metaprogram's OWN code still
+// resolves in `emit.sx` (where `concat`/`print` are flat-imported), so this
+// stays compatible with the 0106 defining-context pin.
+//
+// Comptime-ONLY: `caller_name()` is evaluated at compile time and its value is
+// embedded as a literal in the generated `print(...)` statement — it is never
+// materialized at runtime (so this does NOT exercise issue 0107).
+m :: #import "0738-modules-comptime-arg-caller-context/emit.sx";
+
+caller_name :: () -> string { return "world"; }
+
+main :: () -> s32 {
+    m.emit(caller_name());
+    return 0;
+}

examples/0738-modules-comptime-arg-caller-context/emit.sx

@@ -0,0 +1,9 @@
+// Library metaprogram: builds a `print(...)` statement at compile time from the
+// comptime `$who` value. `concat`/`print` are flat-imported here and resolve in
+// THIS module's context (the 0106 defining-module pin). The substituted caller
+// `$`-arg, by contrast, resolves in the CALLER's context.
+#import "modules/std.sx";
+
+emit :: ($who: string) {
+    #insert concat(concat("print(\"hello ", who), "\\n\");");
+}

examples/expected/0738-modules-comptime-arg-caller-context.exit

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

examples/expected/0738-modules-comptime-arg-caller-context.stderr

@@ -0,0 +1 @@
+

examples/expected/0738-modules-comptime-arg-caller-context.stdout

@@ -0,0 +1 @@
+hello world