cgen: materialise str-def operands in streq pushes

A str `def` has no name(SB) header; the streq push arms read a frame
slot that does not exist. Load the literal through cgexpr (AX=ptr,
BX=len) instead. Both stages.

test/lang/str_def_equality_test.ww

@@ -0,0 +1,20 @@
+// String equality must materialise a module def literal instead of treating
+// its name as a frame-local string header. Use a duplicated local so pointer
+// equality cannot accidentally satisfy the content comparison.
+
+package str_def_equality_test;
+
+import strings;
+
+def expected: str = "dynamic string versus def";
+
+@test fn dynamic_string_and_def_compare_by_content() void = {
+	let observed: str = strings.dup(expected);
+	assert(observed == expected);
+	assert(expected == observed);
+	assert(!(observed != expected));
+	assert(!(expected != observed));
+	let different: str = strings.concat(observed, "!");
+	assert(different != expected);
+	assert(expected != different);
+};