Accept GRAPHIC CLR and a negative DATA item

Two parse handlers refusing something the documentation promises. Landing
together because they are the same defect twice -- a verb's own argument shape
falling through to a general path that cannot see it -- in one file, found by
one program, and verified in one pass.

**`GRAPHIC CLR`** is given as `GRAPHIC mode | CLR` in both docs/06-graphics.md
and docs/11-verb-reference.md, and was refused: `CLR` is a verb of its own, so
the generic arglist path scanned it as a command token and the expression parser
answered "Expected expression or literal". `akbasic_parse_graphic()` takes it as
this verb's keyword argument and emits mode 5 -- which `akbasic_cmd_graphic()`
already treats as "drop the saved shapes and go back to text", so both spellings
are one statement and the exec handler is untouched. The documentation was right
all along; nothing in it changes.

**`DATA -5`** was refused by `akbasic_parse_data()`, and only there: `READ` scans
the source text directly (src/data.c) and always returned the -5 intact. So the
value was right and *reaching* the statement raised -- which, since section 4
settled that `DATA` at run time is a no-op, is what a program does with every
`DATA` line it walks past. A table of coordinates or velocities is full of
negative numbers, which is how a game found it.

The fix accepts a unary minus over a numeric literal and nothing else: `DATA -A#`
is still a mistake worth naming, and `akbasic_leaf_is_literal()` keeps meaning
what it says because other callers rely on it.

tests/read_data.c covers both mechanisms -- reading a negative item and reaching
the line after it -- with a mixed-sign table and a negative float, since the two
were never the same code path.

TODO.md section 9 items 7 and 8, struck.

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

src/verbs.c

@@ -83,7 +83,7 @@ static const akbasic_Verb VERBS[] = {
     { "GETKEY",       AKBASIC_TOK_COMMAND,              -1, NULL,                  akbasic_cmd_getkey },
     { "GOSUB",        AKBASIC_TOK_COMMAND,              -1, NULL,                  akbasic_cmd_gosub },
     { "GOTO",         AKBASIC_TOK_COMMAND,              -1, NULL,                  akbasic_cmd_goto },
-    { "GRAPHIC",      AKBASIC_TOK_COMMAND,              -1, akbasic_parse_arglist, akbasic_cmd_graphic },
+    { "GRAPHIC",      AKBASIC_TOK_COMMAND,              -1, akbasic_parse_graphic, akbasic_cmd_graphic },
     { "GSHAPE",       AKBASIC_TOK_COMMAND,              -1, akbasic_parse_arglist, akbasic_cmd_gshape },
     { "HEADER",       AKBASIC_TOK_COMMAND,              -1, akbasic_parse_optional_arglist, akbasic_cmd_header },
     { "HELP",         AKBASIC_TOK_COMMAND_IMMEDIATE,    -1, NULL,                  akbasic_cmd_help },