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>

docs/v4.0.0/DECOMPOSITION.md

@@ -813,3 +813,4 @@ Addresses are assigned in the node memory map (D-4). Names only here.
 | `REC-ENABLE` | R/W | DoE recorder on/off (`HB-ON` / `HB-OFF`). |
 | `PORT-STATUS` | R | Per-port ready flags, for non-blocking polls. |
 | `NODE-ERROR` | R/W | Arithmetic error flag: set to −1 by `Q./` on division by zero (D-11), by `Q.SQRT` / `Q.LOG` outside their domain (D-12) and by `HOLD` on a bad character or a full buffer (D-13), cleared by writing 0. `VM-ERROR?` reads it. |
+| `CONSOLE-TX` | W | Console transmit: a store sends the low 8 bits of the value as one character. This is what `EMIT` writes to. On the single-node golden model it is a capture register: the character is appended to a buffer on the node and memory is not written (`v4_node_console_attach`, `v4/include/v4/node.h`), so printing words can be run and their output compared with v3's. With the mesh (step 2) it becomes the port to the console node. |