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/include/v4/node.h

@@ -49,6 +49,11 @@
  * build time, which is why the percentage is a macro and not baked in. */
 #define V4_MEM_BOUND V4_GUARD_ELEMS(V4_NODE_WORDS)
 
+/* Characters the console capture holds.  Overridable at build time. */
+#ifndef V4_CONSOLE_CAP
+#define V4_CONSOLE_CAP 4096u
+#endif
+
 typedef struct {
     v4_cell p;                       /* P -- program counter */
     v4_cell a;                       /* A -- address register */
@@ -60,6 +65,13 @@ typedef struct {
     v4_cell mem_guard_head[V4_MEM_BOUND];
     v4_cell mem[V4_NODE_WORDS];
     v4_cell mem_guard_tail[V4_MEM_BOUND];
+
+    /* CONSOLE-TX capture (DECOMPOSITION.md section 7).  See
+     * v4_node_console_attach below. */
+    v4_cell       console_tx;        /* its word address, or -1: no console */
+    unsigned      console_len;       /* characters captured */
+    unsigned      console_dropped;   /* characters that did not fit */
+    unsigned char console[V4_CONSOLE_CAP];
 } v4_node;
 
 /* Zero P, A and B, empty the stacks, and clear memory.  Installs every
@@ -111,4 +123,24 @@ int v4_node_guards_intact(const v4_node *n);
 v4_cell v4_node_load(const v4_node *n, v4_cell addr);
 void    v4_node_store(v4_node *n, v4_cell addr, v4_cell value);
 
+/* CONSOLE-TX, the console's transmit register, on the single-node model.
+ *
+ * EMIT is a device service: one character sent to the console node
+ * (DECOMPOSITION.md 5.10).  The mesh and its ports are development step 2,
+ * and the memory map is open (D-4), so until then the model stands in for
+ * the console with one memory-mapped register and a capture buffer.  That is
+ * enough to run every printing word and compare what it prints with v3.
+ *
+ * After v4_node_console_attach(n, addr), a store to word address `addr`
+ * appends the low 8 bits of the value to n->console instead of writing
+ * memory; n->mem[addr] is never changed by it, and a load from `addr` reads
+ * that memory word as before.  All four store opcodes go through
+ * v4_node_store, so all four reach it.  Once V4_CONSOLE_CAP characters are
+ * held, further ones are counted in n->console_dropped and discarded.
+ *
+ * A node has no console until one is attached: v4_node_reset detaches it
+ * (console_tx = -1) and empties the capture.  Attaching empties it too.
+ * Passing -1 detaches.  The address is the caller's choice. */
+void v4_node_console_attach(v4_node *n, v4_cell addr);
+
 #endif /* V4_NODE_H */