feat(v4.0.0): CONSOLE-TX capture register on the single-node model

EMIT is a device service: a character sent to the console node. The
mesh and its ports are development step 2 and the memory map is open
(D-4), so the single-node model now stands in for the console with one
memory-mapped register, so that printing words can be run and their
output compared with v3's.

v4_node_console_attach(n, addr): after it, a store to word address
`addr` appends the low 8 bits of the value to a buffer on the node
(V4_CONSOLE_CAP characters, 4096 by default) and does not write memory;
a load from `addr` reads the memory word as before. Characters past the
capacity are counted in console_dropped and discarded. The hook is in
v4_node_store, which all four store opcodes use.

It is off by default: v4_node_reset detaches the console (address -1)
and empties the capture, so the ISA's behaviour and every existing test
are unchanged unless a console is attached. The address is the
caller's choice.

New test v4/tests/test_console.c, 22 checks per width: direct stores,
!b, ! and !+ through the executor printing "Hi!", memory at and around
the register untouched, the low byte only, overflow, detach, a console
at word 0, and reset. At 32- and 64-bit cells, optimised and ASan+UBSan
(`make test`, `make sanitize`); all other v4 tests still pass. Four
mutations of the hook each fail.

DECOMPOSITION.md section 7 gains the CONSOLE-TX row. No printing word is
defined here; EMIT and the words on it are still to do.

Test results on the amd64 host only. This is a development check, not
acceptance (JUSTIFICATION.md section 16).

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

v4/README.md

@@ -14,7 +14,8 @@ and every ISA, hosted and bare metal, must still reach its `ok` prompt.
 `make -C v4 test` passing is a development check, not acceptance.
 
 What exists so far is the single node: registers, circular stacks, memory, all
-32 opcodes, and per-opcode and per-call-target heat. `make -C v4 test` builds
+32 opcodes, per-opcode and per-call-target heat, and a console capture
+register (`CONSOLE-TX`) standing in for the console node until the mesh exists. `make -C v4 test` builds
 and runs the tests at both cell widths; `make -C v4 sanitize` repeats them
 under ASan and UBSan. There is no compiler capsule, no POST and no K
 measurement yet.