Move BASIC fixtures into the editable language corpus

Move every program and expectation out of tests/reference and register the unified tests/language corpus as local cases. Remove the old immutable-corpus protections from build, maintenance, and documentation paths.

Co-authored-by: andrew <andrew@aklabs.net>

tests/for_semantics.c

@@ -81,7 +81,7 @@ static void test_counter_survives_the_loop(void)
  * `FOR I = 1 TO 1` executes the body one time on every BASIC there has ever
  * been. Here the entry test treats "the counter has reached the limit" as
  * "do not enter", so the body is skipped entirely -- and
- * tests/reference/language/flowcontrol/forloopwaitingforcommand.bas pins that,
+ * tests/language/flowcontrol/forloopwaitingforcommand.bas pins that,
  * which is why this cannot simply be corrected.
  */
 static void test_single_iteration_loop(void)