fix(0057): clear target_type when lowering variadic pack args

An `xx <int>` argument to a variadic `format`/`print` (a comptime `..$args`
pack) segfaulted when the call was inside an imported-module function. Root
cause: lowerPackCall lowered each pack arg with whatever self.target_type was
set to from the surrounding context. A bare arg is unaffected (inferExprType
ignores target_type), but `xx <expr>`'s result type IS target_type — so
`format("…", xx i)` inside a `-> string` fn cast the int to `string`,
monomorphized __pack_string, and ABI-coerced the 4-byte int as a 16-byte string
fat pointer → corruption. Inline it worked only because target_type was null
there; the imported-module path left it set.

Fix: save/clear/restore self.target_type around the pack-arg lowering loop. A
pack arg is independently typed — comptime `..$args` auto-boxes to Any; a value
pack takes its declared element/protocol type — never a leftover outer target.

examples/242-xx-any-pack-cross-module.sx (+ companion fmt.sx) is the regression.
issues/0057 marked resolved. Unblocks ERR E3.3 (the trace.sx formatter formats
frames with `xx frame`).

Gates: zig build, zig build test, bash tests/run_examples.sh (279 passed; lone
failure is the user's uncommitted 213-canonical-map pack WIP).

examples/242-xx-any-pack-cross-module.sx

@@ -0,0 +1,13 @@
+// Regression for issue 0057: `xx <int>` as a variadic `format` arg inside an
+// imported-module function used to segfault (mis-typed as the enclosing fn's
+// return type, monomorphizing __pack_string + ABI-coercing the int as a
+// 16-byte string fat pointer). Now it auto-boxes to Any like the bare form.
+// The companion module is examples/242-xx-any-pack-cross-module/fmt.sx.
+
+#import "modules/std.sx";
+#import "242-xx-any-pack-cross-module/fmt.sx";
+
+main :: () -> s32 {
+    print("{}", build(3));
+    return 0;
+}

examples/242-xx-any-pack-cross-module/fmt.sx

@@ -0,0 +1,20 @@
+// Companion module for issue-0057 regression. The bug: an `xx <int>` argument
+// to a variadic `format` (a comptime `..$args` pack), inside a function that
+// lives in an IMPORTED module, was mis-typed as the enclosing fn's
+// `target_type` (here `string`) instead of auto-boxing to `Any` — so it
+// monomorphized `__pack_string` and ABI-coerced the 4-byte int as a 16-byte
+// string fat pointer, corrupting memory at runtime. Fixed by clearing
+// `target_type` while lowering pack args.
+
+#import "modules/std.sx";
+
+build :: (n: s32) -> string {
+    result := "items:\n";
+    i : s32 = 0;
+    while i < n {
+        line := format("  item {}\n", xx i);   // <-- the xx-to-Any pack arg
+        result = concat(result, line);
+        i = i + 1;
+    }
+    result;
+}