feat(v4): the shared math in both ROMs, and a mesh node's ROM

MACHINE.md 2.2.  UM*, UM/MOD, D+, DNEGATE, Q.* and Q./ are entries 0-5
of the host ROM and of a mesh node's ROM, from one source (rom/math.f18)
and one entry list (v4/rom_entries.h).  The host ROM starts at entry 18.
The mesh ROM is 277 words; the host ROM 2372.

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

v4/system/boot.c

@@ -6,6 +6,7 @@
  * which only a whole system has.
  */
 #include "v4/post.h"
+#include "v4/rom_entries.h"
 #include "v4/blocks.h"
 #include "v4/boot.h"
 #include "v4/rom_errors.h"
@@ -413,9 +414,9 @@ static int load_capsule(const v4_boot *b, const char *name)
 
 /* The ROM (docs/v4.0.0/MACHINE.md section 2): the machine's one binary,
  * built by v4/tools/mkrom.c for this cell width.  The machine starts at its
- * entry 12, (ROM) (capsules/v4/rom/main.f18), which reads the system's
- * source from the console, assembles it into RAM and, at the source's
- * `start`, runs it. */
+ * entry V4_ROM_START (v4/rom_entries.h), (ROM) (capsules/v4/rom/main.f18),
+ * which reads the system's source from the console, assembles it into RAM
+ * and, at the source's `start`, runs it. */
 #if V4_CELL_BITS == 64
 extern const v4_cell v4_rom_64[];
 extern const unsigned v4_rom_64_words;
@@ -427,7 +428,6 @@ extern const unsigned v4_rom_32_words;
 #define BOOT_ROM       v4_rom_32
 #define BOOT_ROM_WORDS v4_rom_32_words
 #endif
-#define ROM_START      12u               /* the entry where the machine starts */
 #define ROM_STEP_LIMIT 400000000ULL      /* instruction words the ROM may take: it takes about 16 million */
 #define EOT            4                 /* the end of the source: the ROM's (KEY) sees no more */
 
@@ -502,7 +502,7 @@ static int load_system(const v4_boot *b)
     }
 
     v4_node_rom_attach(n, BOOT_ROM, BOOT_ROM_WORDS);
-    n->p = V4_ROM_BASE + (v4_cell)ROM_START;
+    n->p = V4_ROM_BASE + (v4_cell)V4_ROM_START;
     while (n->p >= V4_ROM_BASE) {
         /* the console is given more of the source whenever it has taken
          * all it had */