docs(v4): the ROMs' shared entries, and the mesh ROM as built

DATASHEET-F18.md lists all 19 entries of the host ROM, the six shared ones
with the registers they use.  MACHINE.md 2.2: built; 277 words, not the
estimated 150-200; the system's words are address, push and ;.

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

docs/v4.0.0/MACHINE.md

@@ -9,7 +9,7 @@ section 6 (POST) stands.
 (section 2), which assembles the system's `.f18` from the console; the prebuilt nucleus is gone
 (`a73580e9`, with the final review's fixes in `ff75e69e` and `a28b5f22`). Steps 3 to 6 are not
 built. Hera still sends a node the host's assembled system, not the ROM (step 4, ruled so for
-now). The mesh ROM (section 2.2) is designed and not built.
+now). The mesh ROM (section 2.2) is built.
 
 ## 1. The idea
 
@@ -89,7 +89,7 @@ wiring, drop its odd concepts, add our own. That is what v4 becomes, on the way
 
 ### 2.2 The mesh ROM (designed 2026-10-15)
 
-**Status: designed, for review. Not built.**
+**Status: built** (the commits of plan `docs/v4.0.0/plans/2026-10-15-mesh-rom.md`).
 
 Not every node can carry the ROM of section 2. It is an assembler of about 2100 words, so it
 belongs to host nodes (`JUSTIFICATION.md` §9: "the compiler lives on the host node ... mesh
@@ -119,7 +119,9 @@ by both tables so that the two orders cannot drift apart:
 | 6 to 17 | the assembler's routines, entries 0 to 11 before this change | yes | no |
 | 18 | `(ROM)`, where the host machine starts (entry 12 before this change) | yes | no |
 
-- The mesh ROM is the six entries and the math: about 150 to 200 words.
+- The mesh ROM is the six entries and the math: 277 words (the design estimated 150 to 200;
+  at the ROM's addresses a branch often starts a new instruction word, and a literal takes
+  one of its own). The host ROM grew by the same 277 words, to 2372.
 - Within a build both ROMs sit at the same base, `V4_ROM_BASE`. Code the host assembles that
   calls entry k therefore runs unchanged on a node carrying either ROM.
 - `v4/tools/mkrom.c` builds both ROMs at each cell width. `v4/tools/mkmap.c` gives each entry's
@@ -127,8 +129,12 @@ by both tables so that the two orders cannot drift apart:
 - The renumbering costs nothing: until now nothing but the boot and the tests' harness used an
   entry number, and both only to start the machine.
 
-**The system calls the ROM.** `UM*`, `UM/MOD`, `D+`, `DNEGATE`, `Q.*` and `Q./` keep their
-dictionary entries in the system. Each one's code becomes a single `jump` to its ROM entry.
+**The system calls the ROM.** `UM*`, `UM/MOD`, `D+`, `DNEGATE`, `Q.*` and `Q./` stay words of
+the system, with the dictionary entries they had (`UM/MOD` never had one: it is a word of the
+`.f18` layer, and FORTH-79's `U/MOD` is built on it). Each one's code is its entry's address,
+`push` and `;`: the address goes onto the return stack and `;` goes there, which is a jump. A
+branch to a constant is not in the assembler's language (`v4/src/text.c`), and the ROM's
+assembler must accept the same language.
 Every other word (`*`, `/MOD`, `U*`, number output, the other Q48 words) calls them by name, as
 now, and is unchanged. `Q.TO-INT` stays an ordinary system word; the ROM keeps its own copy of
 that one-line shift rather than making it an entry. `dict_hash` and `rom_hash` change, and each
@@ -144,7 +150,7 @@ products have no births yet.
   values: 0, -1, the most negative cell, the largest cell, `UM/MOD` at the edge `uhi < ud`, and
   `Q./` by zero (error 11 and D-11's result).
 - The mesh ROM holds the six entries and the math, and nothing of the assembler.
-- Hera's unit test: a node she has born computes `UM*`, `UM/MOD` and `Q./` and prints the right
+- Hera's unit test: a node she has born computes `UM*`, `U/MOD` (on `UM/MOD`) and `Q./` and prints the right
   answers.
 - Then the whole suite at both widths, POST 538 of 538, and the three QEMU boots, as
   `.claude/CLAUDE.md` says.