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>

capsules/BLOCK_MAP.md

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