test: add struct/pointer/func interpolation fail tests (#1499)

Author: SchoolyB

TESTING.md

@@ -56,7 +56,7 @@ Integration tests compile and run `.ez` programs end-to-end through the full com
 - `integration-tests/pass/stdlib/` — 29 stdlib module tests
 - `integration-tests/pass/warnings/` — 22 warning detection tests
 - `integration-tests/pass/multi-file/` — 31 multi-file import tests (basic, alias, structs, enums, constants, private visibility, transitive, circular, collision, nested, struct functions, imported struct with enum field)
-- `integration-tests/fail/errors/` — 495 error detection tests
+- `integration-tests/fail/errors/` — 498 error detection tests
 - `integration-tests/fail/multi-file/` — 20 multi-file error detection tests (collision, private access, type mismatch, undefined module, cross-file error attribution)
 
 **Running:**

integration-tests/fail/errors/E3041_func_ref_in_interpolation.ez

@@ -0,0 +1,18 @@
+/*
+ * Error Test: E3041 - func-typed variable in string interpolation
+ *
+ * Regression for #1499: "${f}" where f is a func-typed variable
+ * holding a function reference used to leak to clang. Typechecker
+ * now rejects with a hint to call the function or format its result.
+ * func resolves as TK_UNKNOWN with name "func", so the check keys
+ * off the name rather than the kind.
+ */
+
+do add(a int, b int) -> int {
+    return a + b
+}
+
+do main() {
+    mut f func = ()add
+    println("fn: ${f}")
+}

integration-tests/fail/errors/E3041_pointer_in_interpolation.ez

@@ -0,0 +1,13 @@
+/*
+ * Error Test: E3041 - pointer value in string interpolation
+ *
+ * Regression for #1499: "${ptr}" where ptr is ^T used to leak to
+ * clang the same way as structs did. Typechecker now rejects with
+ * a hint to dereference with ^ or format the pointee.
+ */
+
+do main() {
+    mut v int = 42
+    mut p ^int = addr(v)
+    println("ptr: ${p}")
+}

integration-tests/fail/errors/E3041_struct_in_interpolation.ez

@@ -0,0 +1,19 @@
+/*
+ * Error Test: E3041 - struct value in string interpolation
+ *
+ * Regression for #1499 (bug I reported this session). "${p}" where
+ * p is a struct used to fall through codegen's default %lld + (long
+ * long) cast and produce a raw clang diagnostic. Typechecker now
+ * rejects at the interpolation site with a hint pointing at
+ * per-field formatting.
+ */
+
+const Point struct {
+    x int
+    y int
+}
+
+do main() {
+    mut p Point = Point{x: 3, y: 4}
+    println("point: ${p}")
+}