Compiler: fix 16 of 18 compile errors — 32/72 tests passing

Fixes:
- Include graphics.h/virmem.h in gwrt.h (5 programs: draw_commands,
  get_put, graphics_stubs, peek_gfx, view_window)
- RND(n): discard argument to buffer instead of emitting it inline
- Float constant 0: emit "0.0f" not "0f" (invalid C suffix)
- MID$ function: proper 3-argument emission with optional length
- FOR/NEXT: use static variables instead of block scope for limit/step
  (fixes FOR inside IF/THEN and unmatched brace issues)
- RESUME/RESUME NEXT: handle as statements (not misparse as FOR/NEXT)
- ON TIMER/KEY/COM: skip event trap setup (not misparse as ON n GOTO)
- MID$ assignment: recognize and skip (don't misparse as variable)
- ERROR statement: emit gw_error() call
- ERASE: stub handler

Only 2 compile errors remain: bubble_sort (SWAP with arrays),
misc_stmts (ENVIRON$ variable name clash).

32 of 72 tests pass. New: datetime, luhn, peek_poke, error_handler,
on_timer, timer_stop, monte_carlo, number_guess.

include/gwrt.h

@@ -14,6 +14,8 @@
 #include "strpool.h"
 #include "portio.h"
 #include "sound.h"
+#include "graphics.h"
+#include "virmem.h"
 #include <setjmp.h>
 
 /* Initialization / shutdown */