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/assets/README.md

@@ -0,0 +1,22 @@
+# tests/assets
+
+Fixtures for the tests that need a real file on disk. Only one so far.
+
+## sprite8x8.png
+
+An 8x8 solid magenta square, 76 bytes, generated rather than drawn -- it is a
+PNG header, one IDAT of a single repeated RGBA pixel, and an IEND.
+
+It exists for one assertion in `tests/akgl_backends.c`: that
+`SPRSAV "assets/sprite8x8.png", 1` reaches SDL_image, decodes, and lands on the
+render target as magenta pixels at the coordinate `MOVSPR` put it. Magenta
+because nothing else in that test file draws in it, so a stray pixel cannot be
+mistaken for this one.
+
+Eight by eight rather than a Commodore sprite's 24x21, deliberately: a sprite
+loaded from a file takes the image's own size, and a fixture that happened to be
+24x21 could not tell that apart from a hardcoded constant.
+
+libakgl's own fixtures under `deps/libakgl/tests/assets/` are borrowed where they
+fit -- the monospaced font is used from there rather than copied. There was no
+image small enough to be worth borrowing: the smallest is a 576x384 spritesheet.