Refuse a branch to a line the program did not number

GOTO 100 in a script written without line numbers finds the hundredth line
and branches there. Silent, plausible and wrong: the test for it loops
forever printing the second line when the check is removed.

akbasic_runtime_check_targets() is a fourth prescan beside the label, DATA
and TYPE ones, run on every entry into MODE_RUN -- the earliest the check
can be made and the only place all four ways a program arrives pass
through. A target naming an empty line is still allowed, for the same
reason RENUMBER leaves one alone, and a fully numbered program is
unaffected, which is every program that existed before this.

It shares RENUMBER's walk rather than repeating it. renumber.c grows an
akbasic_TargetWalk -- a self pointer and a visit function -- and
rewrite_line() takes one. RENUMBER's visitor substitutes the number a line
moved to; the check's substitutes the number unchanged and raises. One
walk, so the two cannot disagree about what a branch target is.

The check points environment->lineno at the line being walked so the "? N :"
prefix names the offending line. The other three prescans do not and report
whichever line the loader stopped on; TODO.md section 5 item 64 records it.

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

src/runtime.c

@@ -380,7 +380,7 @@ akerr_ErrorContext *akbasic_runtime_set_mode(akbasic_Runtime *obj, int mode)
      */
     if ( obj->mode == AKBASIC_MODE_RUN && obj->environment != NULL ) {
 	/*
-	 * All three prescans, and all three inside one ATTEMPT.
+	 * All four prescans, and all four inside one ATTEMPT.
 	 *
 	 * **A prescan failure is the program's mistake, not the host's**, so it
 	 * has to leave here as a BASIC error line rather than as a raised
@@ -417,6 +417,16 @@ akerr_ErrorContext *akbasic_runtime_set_mode(akbasic_Runtime *obj, int mode)
 	     * skipped the lines that declared it.
 	     */
 	    CATCH(errctx, akbasic_structtype_scan(obj));
+	    /*
+	     * Then the branch targets, last because it is the only one that reads
+	     * nothing into the runtime -- it only refuses. A loaded line may be
+	     * *given* its number rather than carry one, so `GOTO 100` in a script
+	     * written without numbers would otherwise find the hundredth line and
+	     * branch there, which is plausible, silent and wrong. Said here rather
+	     * than at the branch, because before the program runs is earlier and
+	     * every path into a run comes through this function.
+	     */
+	    CATCH(errctx, akbasic_runtime_check_targets(obj));
 	} CLEANUP {
 	} PROCESS(errctx) {
 	} HANDLE_DEFAULT(errctx) {