Finish the language: every remaining verb group, and the defects that blocked them

Closes groups A, C, D, E, F, H and J of TODO.md section 4, plus RESTORE and
RENUMBER, and closes section 6 -- all seventeen reference defects. Seven of
those turned out to have been fixed or never ported and nobody had written it
down; the audit records the evidence for each.

Two of the seventeen were real. math_plus mutated its left operand when the
operand was mutable, so A# + 1 could modify A#; it was gated on FOR/NEXT
coverage because NEXT relied on the mutation, so tests/for_next.c came first
and NEXT now writes the counter back itself. And the binary operators summed
both numeric fields of their right operand, which no BASIC program can reach
-- that one needed a test written against the value API.

Writing the tests turned up eight defects nobody had listed. Seven are fixed:

  IF A = 2 THEN was a parse error; only == worked
  IF ... AND ... was a parse error, because a condition parsed as one relation
  IF A = 1 OR B = 2 THEN was silently always false, and so was IF A THEN
  EXIT before any NEXT restarted the program and exhausted the variable pool
  READ never found a DATA line above it, and swallowed the lines between
  PRINT 2 + 2 at the prompt was filed as program text instead of answering
  a short read discarded its bytes, so COPY produced empty files
  every verb taking an argument list said "peek() returned nil token!" on none

The eighth is not fixed and cannot be quietly: a FOR whose step overshoots
runs its body one extra time, and FOR I = 1 TO 1 runs it zero times. The two
errors cancel for a step of 1, which is why neither was noticed. Correcting
them changes the expected output of a checked-in acceptance file, and
tests/reference/README.md forbids editing one to suit this interpreter. It is
tests/for_semantics.c in AKBASIC_KNOWN_FAILING_TESTS, asserting the correct
contract, and TODO.md items 19 and 20.

Sprites are real libakgl actors with a renderfunc of their own, because
akgl_actor_render draws every sprite square and an actor has no per-axis
scale. Both are filed upstream. SPRSAV takes an image file, an SSHAPE handle
or a 63-element integer array -- a string here cannot hold a zero byte.

Verbs that need hardware that does not exist are refused by name with the
reason rather than faked: SYS, HEADER, COLLECT, BACKUP, BOOT, FILTER, and
DIRECTORY, which is refused for a missing libakstdlib wrapper filed upstream.

94 tests in the default build, 93 with SDL, 94 under ASan and UBSan, doxygen
clean. The Go acceptance corpus stayed green throughout.

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

tests/value_arithmetic.c

@@ -191,17 +191,22 @@ int main(void)
     TEST_REQUIRE_STR(out->stringval, "ababab");
 
     /*
-     * mathPlus mutates self in place when self is mutable, where every other
-     * operator always clones. CommandNEXT's loop increment depends on it --
-     * TODO.md section 12 item 4. Pinning the asymmetry so a "cleanup" fails here
-     * rather than silently breaking every FOR loop.
+     * math_plus clones like every other operator, mutable operand or not.
+     *
+     * This used to assert the opposite. The reference mutates `self` in place
+     * when it is mutable, which made `A# + 1` modify `A#` whenever the left
+     * operand came out of a variable -- TODO.md section 6 item 4. It was left
+     * alone because NEXT's loop increment relied on the mutation to advance the
+     * counter; NEXT writes the result back itself now, so the asymmetry is gone
+     * and this asserts that it stays gone.
      */
     set_int(&A, 10);
     A.mutable_ = true;
     set_int(&B, 5);
     TEST_REQUIRE_OK(akbasic_value_math_plus(&A, &B, &SCRATCH, &out));
-    TEST_REQUIRE(out == &A, "math_plus on a mutable value must return self, not the scratch");
-    TEST_REQUIRE_INT(A.intval, 15);
+    TEST_REQUIRE(out == &SCRATCH, "math_plus must use the scratch even for a mutable operand");
+    TEST_REQUIRE_INT(A.intval, 10);
+    TEST_REQUIRE_INT(SCRATCH.intval, 15);
 
     set_int(&A, 10);
     A.mutable_ = false;
@@ -209,6 +214,34 @@ int main(void)
     TEST_REQUIRE(out == &SCRATCH, "math_plus on an immutable value must use the scratch");
     TEST_REQUIRE_INT(A.intval, 10);
 
+    /*
+     * The unused numeric field is ignored, not added in.
+     *
+     * The reference reads a right-hand operand as `intval + int64(floatval)`,
+     * which gives the right answer only for as long as whichever field is unused
+     * happens to be zero -- TODO.md section 6 item 5, filed as a landmine
+     * because no program can reach it. It is reachable from here: a value is a
+     * plain struct and nothing stops one carrying both. Without this the fix is
+     * untested, and the old code passes every other test in the tree.
+     */
+    set_int(&A, 2);
+    set_int(&B, 5);
+    B.floatval = 3.0;			/* stale, from whatever B held before */
+    TEST_REQUIRE_OK(akbasic_value_math_plus(&A, &B, &SCRATCH, &out));
+    TEST_REQUIRE_INT(out->intval, 7);
+
+    set_float(&A, 2.0);
+    set_float(&B, 5.0);
+    B.intval = 3;			/* stale the other way round */
+    TEST_REQUIRE_OK(akbasic_value_math_plus(&A, &B, &SCRATCH, &out));
+    TEST_REQUIRE_FEQ(out->floatval, 7.0);
+
+    /* And a truth value used as a number is -1, which is what AND/OR rely on. */
+    set_int(&A, 0);
+    TEST_REQUIRE_OK(akbasic_value_set_bool(&B, true));
+    TEST_REQUIRE_OK(akbasic_value_math_plus(&A, &B, &SCRATCH, &out));
+    TEST_REQUIRE_INT(out->intval, -1);
+
     /* Type errors. */
     set_string(&A, "text");
     set_int(&B, 1);