Fix five defects the port uncovered in the reference parser and scanner

Chained subtraction and exponentiation fold left instead of abandoning the
rest of the line -- `1 - 2 - 3` gave -1 and dropped the `- 3`. A unary leaf's
operand moves to .left, so a negative literal is one argument again and a
second argument no longer overwrites it. An operator in a line's final column
survives. Hex literals reach the parser whole. A leading zero is padding, not
a radix.

Item 16 stays pinned: the fix works and costs an upstream golden case, which
is a decision rather than a defect.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/known_reference_defects.c

@@ -10,72 +10,37 @@
  * When one of these is fixed, this test starts passing and CTest reports
  * "unexpectedly passed". That is the cue to split the fixed assertion out into
  * the corresponding suite in AKBASIC_TESTS and strike the item from TODO.md
- * section 12.
+ * section 6.
+ *
+ * Five of the six this file was written with are gone that way. What is left is
+ * the one whose fix is not a coding question but a decision, and it is described
+ * where it is asserted.
  */
 
 #include "harness.h"
 
 int main(void)
 {
-    akbasic_ASTLeaf *leaf = NULL;
-    akbasic_Value *out = NULL;
-    char rendered[AKBASIC_MAX_STRING_LENGTH];
-
     TEST_REQUIRE_OK(harness_start(NULL));
 
     /*
-     * 12.12 -- subtraction stops after one operator, so `1 - 2 - 3` parses as
-     * `1 - 2` and abandons the rest of the line. It should fold left, the way
-     * addition does.
-     */
-    TEST_REQUIRE_OK(harness_parse("A# = 1 - 2 - 3", &leaf));
-    TEST_REQUIRE_OK(akbasic_leaf_to_string(leaf, rendered, sizeof(rendered)));
-    TEST_REQUIRE_STR(rendered, "( A# (- (- 1 2) 3))");
-
-    /*
-     * 12.13 -- a unary-minus argument inflates the arity count, because the
-     * counter walks .right and a unary leaf keeps its operand there. ABS(-9)
-     * should be one argument.
-     */
-    TEST_REQUIRE_OK(akbasic_environment_zero(HARNESS_RUNTIME.environment));
-    TEST_REQUIRE_OK(harness_parse("A# = ABS(-9)", &leaf));
-    TEST_REQUIRE_OK(akbasic_runtime_evaluate(&HARNESS_RUNTIME, leaf, &out));
-    TEST_REQUIRE_INT(out->intval, 9);
-
-    /*
-     * 12.14 -- a comparison operator in the final column of a line is dropped,
-     * because matchNextChar returns without setting a token type when it cannot
-     * peek past the end.
-     */
-    TEST_REQUIRE_OK(akbasic_scanner_scan(&HARNESS_RUNTIME, "A# =", NULL, 0));
-    TEST_REQUIRE_INT(HARNESS_RUNTIME.environment->nexttoken, 2);
-
-    /*
-     * 12.15 -- hex literals do not survive the scanner. matchNumber lets 'x'
-     * through but not the digits after it, so the parser's base-16 branch is
-     * unreachable.
-     */
-    TEST_REQUIRE_OK(akbasic_scanner_scan(&HARNESS_RUNTIME, "PRINT 0xff", NULL, 0));
-    TEST_REQUIRE_STR(HARNESS_RUNTIME.environment->tokens[1].lexeme, "0xff");
-
-    /*
-     * 12.16 -- "Reserved word in variable name" never fires, because the lexeme
-     * still carries its type suffix when the verb table is searched. PRINT$
-     * should be refused as a variable name.
+     * 6.16 -- "Reserved word in variable name" never fires, because the lexeme
+     * still carries its type suffix when the verb table is searched, so PRINT$
+     * misses every table and becomes an ordinary string variable.
+     *
+     * Still here rather than fixed, and the reason is worth knowing before
+     * anybody "just fixes it": stripping the suffix before the lookup is four
+     * lines and it works, and it costs a golden case. The reference's own
+     * examples/strreverse.bas names a variable INPUT$ -- which a real C128 would
+     * refuse too, so the reference program is the wrong one. But the corpus is
+     * the acceptance contract and it lives in a submodule this repository may
+     * not edit, so the diagnostic and the corpus cannot both be right. TODO.md
+     * section 6 records the trade rather than making it silently.
      */
     HARNESS_RUNTIME.hasError = false;
     TEST_REQUIRE_OK(akbasic_scanner_scan(&HARNESS_RUNTIME, "PRINT$ = 1", NULL, 0));
     TEST_REQUIRE(HARNESS_RUNTIME.hasError, "PRINT$ should be refused as a variable name");
 
-    /*
-     * 12.10 -- a leading zero selects base 8, so `PRINT 010` prints 8 and
-     * `PRINT 08` will not parse. Commodore BASIC has no octal literals.
-     */
-    TEST_REQUIRE_OK(akbasic_environment_zero(HARNESS_RUNTIME.environment));
-    TEST_REQUIRE_OK(harness_parse("A# = 010", &leaf));
-    TEST_REQUIRE_OK(akbasic_runtime_evaluate(&HARNESS_RUNTIME, leaf, &out));
-    TEST_REQUIRE_INT(out->intval, 10);
-
     harness_stop();
     return akbasic_test_failures;
 }