feat(v4.0.0): a node is sent text as a message and sends back what it prints

MESH.md step 3.  The ports are the transport; the message is what is
transported: to, from, type, heat and TTL, ACL tag, sequence, length, then
text four characters to a word.

- quit.v4: a node with nothing to do is blocked reading "any port"; text
  for it is interpreted; (FINISH) sends what it printed and then how the
  text ended, and it waits again
- core.v4: EMIT keeps what is printed, (FLUSH-OUT) and (HDR) send it to the
  sender on the port the message came on.  EMIT still needs one free data
  cell and no more; it works on the return stack and in A and B
- message.h/.c: the same format for whatever is on a port and is not a node
- boot.c: the boot is the node's console on port 1 and its kernel on port 0
- the prompt tests are a console that speaks messages
- gone: v4_line_begin, v4_line_done, v4_line_status; writing a node's input
  buffer and setting its P from outside; any use of CONSOLE-TX

Verified: make -C v4 test (test_host_quit.c 1283 checks, the full-stack
figures unchanged) and make -C v4 sanitize pass; hosted-check passes on
three ISAs with POST 550 of 550; clean qemu with STARFORTH_V4=1 passes POST
and answers lines typed at each prompt on amd64, aarch64 and riscv64
(logs/20261006-110551, -111621, -111341).  -110837 is an aarch64 run ended
by the test wrapper's limit while still in UEFI firmware; it shows nothing
about v4.

Not done: KEY, EXPECT and QUERY still read the console's input registers;
a message not for this node is let go (step 4).

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

v4/README.md

@@ -59,7 +59,7 @@ State, 2026-10-05, after `ENGINE.md` step 1: all six builds start the nucleus,
 load `v4:forth79.4th`, pass POST and reach `ok>`, with the same lines:
 
 ```
-PARITY:V4_NUCLEUS name=v4:nucleus-64.f18 capsule_id=... capsule_hash=0xbcc90cc5a638a299 words=298
+PARITY:V4_NUCLEUS name=v4:nucleus-64.f18 capsule_id=... capsule_hash=0x97a655081b2af559 words=298
 PARITY:V4_CAPSULE name=v4:forth79.4th capsule_id=... dict_hash=...
 PARITY:V4_POST tests=550 pass=550 fail=0
 PARITY:V4_CAPSULE name=v4:post79.4th capsule_id=... dict_hash=...
@@ -68,23 +68,23 @@ POST: PASSED
 ok>
 ```
 
-Bare-metal logs: `logs/20261006-102421/amd64/`, `logs/20261006-102706/aarch64/`,
-`logs/20261006-103048/riscv64/`. On each, seven lines were then typed at the
+Bare-metal logs: `logs/20261006-110551/amd64/`, `logs/20261006-111621/aarch64/`,
+`logs/20261006-111341/riscv64/`. On each, seven lines were then typed at the
 prompt through the serial port — a definition, its use with the capsule's
 `U*`, `COLD`, the capsule word again, a `FORGET` of it (refused), a kernel
 word no one serves, and `BYE` — and each was answered as the hosted binary
 answers it. The
 capsules were unsigned (no signing key on this machine).
 
-**A node is handed a line** (`docs/v4.0.0/ENGINE.md` 3.1). It does not read
-its own command line and prints no prompt. Its host puts the text in the
-node's input buffer and starts it at `(LINE)`; the node interprets the text
-and stops at `(IDLE)`, leaving in `(LINE-STATUS)` how it ended. The host says
-` ok` or ` ERROR` and prompts, as the kernel's REPL does for a v3 VM. A line
-may be 1024 characters, a block. `v4_line_begin`, `v4_line_done` and
-`v4_line_status` (`v4/include/v4/image.h`) are that interface;
-`v4_boot_line` (`v4/system/boot.c`) is the one loop the hosted binary, the
-kernel and the capsule loader all run it with.
+**A node is sent text as a message** (`docs/v4.0.0/MESH.md` section 6). It
+does not read its own command line, prints no prompt and has no console of
+its own. With nothing to do it is blocked reading its ports. A neighbour
+writes it a message; text for it is interpreted; what it prints goes back
+as messages to the sender, and then a message saying how the text ended.
+Its console says ` ok` or ` ERROR` and prompts, as the kernel's REPL does
+for a v3 VM. A message carries 1024 characters, a block. `v4/src/message.c`
+is the format for a host; `v4_boot_line` (`v4/system/boot.c`) is the
+console both products and the capsule loader use.
 
 **A node asks its kernel by writing to its port** (`ENGINE.md` 3.3). The
 write blocks the node until it has been served. A kernel word is a