feat(v4): mkrom -- the ROM built off the node; it reads tokens

The ROM's source is F18 code in capsules/v4/rom, assembled off the node by
v4/tools/mkrom.c with the memory map's constants from v4/tools/mkmap.c
(MACHINE.md 2).  main.f18 is the entry table payloads call through (2.1);
in.f18 reads the console and splits it into tokens exactly as text.c does,
the token held packed as a counted string.  The ROM's state is a reserved
area of the map (host_map.h ROM_*).  text.c gains the constant directive,
so the off-node assembler and the ROM speak one language.  The map also
names (NUMBER)'s state, for the next task.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

v4/include/v4/text.h

@@ -46,6 +46,9 @@
  *                    may use other macros; it may not define a word, a label
  *                    or a macro.
  *
+ *   constant NAME VALUE
+ *                    NAME is the number VALUE, as v4_text_constant gives it.
+ *
  *   header NAME      a dictionary entry for NAME (capsule/dict.v4), laid down
  *                    in front of the word that the next `:` starts, and
  *                    linked to the header before it.  NAME is taken as it