Give argument lists their own link, not each argument's .right

An identifier's subscript list, a unary's operand and a binary's right-hand
side all lived in the same field the argument chain used, so ABS(-9),
MOD(A#, B# + 12) and any array reference in a parameter list were counted as
extra arguments and refused. Subscript lists move to .expr and arguments now
chain through a dedicated .next.

This is the third time the same collision was fixed; the first two moved it
along rather than removing it.

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

include/akbasic/grammar.h

@@ -106,6 +106,26 @@ typedef struct akbasic_ASTLeaf
     struct akbasic_ASTLeaf *left;
     struct akbasic_ASTLeaf *right;
     struct akbasic_ASTLeaf *expr;
+    /**
+     * The next argument or subscript in a list, and nothing else.
+     *
+     * **This field exists because reusing `.right` for it was a bug.** The
+     * reference chains an argument list through each argument's `.right`
+     * (`basicparser.go`'s argumentList), which is also where a unary leaf keeps
+     * its operand, where a binary leaf keeps its right-hand side, and where an
+     * identifier used to keep its subscript list. The three meanings were
+     * indistinguishable, and every one of them produced a wrong answer:
+     *
+     *     ABS(-9)             counted as two arguments, refused
+     *     MOD(A#, B# + 12)    counted as three, refused
+     *     MOD(-7, 3)          the second argument overwrote the first's operand
+     *
+     * -- TODO.md section 6 item 13, and the "array references in parameter
+     * lists" item in section 4, which turned out to be the same defect twice.
+     * An argument list's *head* is still the list leaf's `.right`; only the
+     * sibling chain moved here.
+     */
+    struct akbasic_ASTLeaf *next;
 } akbasic_ASTLeaf;
 
 /**