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/include/v4/rom_entries.h

@@ -0,0 +1,31 @@
+/* rom_entries.h -- the ROMs' shared entries (docs/v4.0.0/MACHINE.md 2.2).
+ *
+ * Both ROMs, the host node's and a mesh node's, begin with these entries, in
+ * this order: entry i is at V4_ROM_BASE + i, a jump to the routine.  A
+ * payload calls an entry, never an address inside the ROM.  This is the one
+ * list: tools/mkmap.c makes of it the (MATH-ENTRIES) macro both ROMs' entry
+ * tables begin with (capsules/v4/rom/main.f18, mesh.f18) and the constants
+ * ROM-UM* ... the system's source calls them by (tests/host_map.h).
+ *
+ * X(INDEX, ROUTINE, FORTH, EFFECT): ROUTINE is the ROM's name for the code
+ * (capsules/v4/rom/math.f18), FORTH the system's word that calls it, EFFECT
+ * its stack effect.  An entry is added only at the end.
+ */
+#ifndef V4_ROM_ENTRIES_H
+#define V4_ROM_ENTRIES_H
+
+#define V4_ROM_MATH_ENTRIES(X) \
+    X(0, "(UM*)",     "UM*",     "( u1 u2 -- ulo uhi )") \
+    X(1, "(UM/MOD)",  "UM/MOD",  "( ulo uhi ud -- urem uquot )") \
+    X(2, "(D+)",      "D+",      "( d1 d2 -- d3 )") \
+    X(3, "(DNEGATE)", "DNEGATE", "( d -- -d )") \
+    X(4, "(Q.*)",     "Q.*",     "( a b -- c )") \
+    X(5, "(Q./)",     "Q./",     "( a b -- q )")
+
+#define V4_ROM_MATH_COUNT 6u
+
+/* The host ROM's own entries follow: the assembler's twelve routines, then
+ * (ROM), where the host machine starts (capsules/v4/rom/main.f18). */
+#define V4_ROM_START (V4_ROM_MATH_COUNT + 12u)
+
+#endif