Implement the housekeeping verbs, and stop the REPL spinning after an error

NEW, CLR, CONT, SWAP, TRON, TROFF and HELP. None is in the Go reference, so
what each means here is recorded beside it.

Testing CONT turned up a hang that predates this: the runtime's error class is
deliberately sticky, and with run_finished_mode REPL every later step
re-entered REPL and printed READY -- overwriting the QUIT that end of input
had just set. An interactive session that hit one runtime error printed READY
forever instead of exiting.

RESTORE and RENUMBER are deferred with reasons; RESTORE turns out to need a
DATA pointer this port does not have, which is a defect in its own right.

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

CMakeLists.txt

@@ -130,6 +130,7 @@ set(AKBASIC_SOURCES
   src/runtime_commands.c
   src/runtime_functions.c
   src/runtime_graphics.c
+  src/runtime_housekeeping.c
   src/runtime_input.c
   src/scanner.c
   src/sink_stdio.c
@@ -239,6 +240,7 @@ set(AKBASIC_TESTS
   grammar_leaves
   graphics_verbs
   hostvars
+  housekeeping_verbs
   input_verbs
   numeric_contract
   parser_commands