fix(stdlib/E4): source-pin pack-fn fixed-prefix param types to the defining module

E4's pack-fn source-pin was incomplete: an imported pack function's
fixed-prefix (non-pack) parameter types were resolved in the CALLER's
module, so a param whose type is bare-visible only in the pack fn's own
module was wrongly rejected with "type 'X' is not visible" — even though
the equivalent plain fn (typed via the source-pinned call-arg path) ran
fine.

Two sites in the pack-mono path re-resolved the fixed-prefix param type
in the caller's context:
  - lowerPackFnCall: the call-site arg-typing pass (to contextually type
    the arg from its param) — fires first.
  - monomorphizePackFn: the body parameter binding, after the caller
    source was restored from the signature build.

Both now resolve via resolveParamTypeInSource(fd.body.source_file, &p),
pinning to the pack fn's defining module — matching the already-pinned
signature build, the body lowering, and the cross-module call-arg typing
sites. The call-site arg itself is still lowered AFTER, in the caller's
context (issue 0106).

Regression: examples/0544-packs-imported-pack-fn-fixed-param-source-pin
(main -> lib -> dep; `Needs` two flat hops away, never named in main).
Fails pre-fix with "type 'Needs' is not visible"; passes after. A control
plain fn in the same lib already ran, isolating the pack-mono path.

examples/0544-packs-imported-pack-fn-fixed-param-source-pin.sx

@@ -0,0 +1,22 @@
+// Regression (stdlib E4): an imported pack function whose fixed-prefix param
+// type is visible only in its defining module must resolve during pack
+// monomorphization. `lib.sx` imports `dep.sx` (which defines `Needs`) and
+// exposes `make() -> Needs` plus `use_pack(n: Needs, ..$args)`. `main` imports
+// ONLY `lib.sx`, so `Needs` is two flat hops away and not bare-visible here —
+// main never names it.
+//
+// Before the fix, `monomorphizePackFn` restored the caller's source before
+// re-binding the fixed-prefix params, so `n: Needs` was resolved in main's
+// context and rejected with "type 'Needs' is not visible" — even though the
+// control plain fn `use_plain(n: Needs)` (typed via the source-pinned call-arg
+// path) ran fine. The fixed-prefix param is now resolved under the pack fn's own
+// source (`fd.body.source_file`), matching the rest of the pack signature/body.
+#import "modules/std.sx";
+#import "0544-packs-imported-pack-fn-fixed-param-source-pin/lib.sx";
+
+main :: () -> s32 {
+    x := make();
+    print("plain {}\n", use_plain(x));
+    print("pack {}\n", use_pack(x, 1, 2));
+    return 0;
+}

examples/0544-packs-imported-pack-fn-fixed-param-source-pin/dep.sx

@@ -0,0 +1,3 @@
+// `Needs` lives two flat-import hops away from `main` (main -> lib -> dep), so
+// it is NOT bare-visible at the call site under non-transitive visibility.
+Needs :: struct { v: s64; }

examples/0544-packs-imported-pack-fn-fixed-param-source-pin/lib.sx

@@ -0,0 +1,16 @@
+// `lib.sx` imports `dep.sx`, so `Needs` is bare-visible HERE. A module that
+// imports only `lib.sx` cannot see `Needs` (non-transitive). The pack fn's
+// fixed-prefix param `n: Needs` must therefore resolve in this module's
+// context, not the caller's.
+#import "modules/std.sx";
+#import "dep.sx";
+
+make :: () -> Needs => Needs.{ v = 7 };
+
+// Control: a plain (non-pack) fn with the same fixed param already resolves in
+// its defining module — the cross-module call-arg typing path is source-pinned.
+use_plain :: (n: Needs) -> s64 => n.v;
+
+// Pack fn: the fixed-prefix param `n: Needs` is bound during pack
+// monomorphization. Its type must resolve under this module's source.
+use_pack :: (n: Needs, ..$args) -> s64 => n.v + args[0];

examples/expected/0544-packs-imported-pack-fn-fixed-param-source-pin.exit

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

examples/expected/0544-packs-imported-pack-fn-fixed-param-source-pin.stderr

@@ -0,0 +1 @@
+

examples/expected/0544-packs-imported-pack-fn-fixed-param-source-pin.stdout

@@ -0,0 +1,2 @@
+plain 7
+pack 8