feat(v4.0.0): nodes that talk -- ports, a node born empty, and the fabric

MESH.md step 1, in the engine, which knows nothing of StarForth or of any
kernel.

- node: V4_PORTS ports (8), a build parameter; "any port" and the port the
  last such read came from; a read blocks until the neighbour writes, as a
  write blocks until the neighbour reads; v4_node_born: empty, P at "any
  port"
- exec: a fetch from a port -- @ @b @+ @p, or of an instruction word when P
  is a port -- waits for a word; a node executes what arrives at a port
  without advancing P; a blocked node goes on from the slot it stopped at
- fabric: the nodes there are and the table of how their ports are wired,
  both changed while the nodes run; devices on a port; asleep and awake; a
  step is every unblocked node executing one instruction word, then every
  write with a reader waiting being handed over
- DECOMPOSITION.md section 6: four named ports withdrawn for V4_PORTS
  numbered ones and wiring as data, as ruled

Verified: tests/test_fabric.c, 53 checks at both widths: two nodes exchange
words; an empty node is filled through its port by a device, and by another
node, and runs what it was sent; a word is passed on by a node in between;
a waiting node executes nothing; the wiring is changed while they run; a
node is put to sleep, woken and removed while looping; a node is born while
others run; the fabric is given more room.  make -C v4 test and make -C v4
sanitize pass.  The single-node products are unchanged: hosted-check on
three ISAs, and clean qemu with STARFORTH_V4=1 on amd64, aarch64 and
riscv64 with lines typed at each prompt (logs/20261006-074907, -075150,
-075532).

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

v4/tests/host_map.h

@@ -68,7 +68,7 @@
 #define EMIT_HOOK    (BUF0_W - 7)               /* the xt of a word EMIT gives its character to, or 0: the console */
 #define CATCH        (BUF0_W - 8)               /* non-zero: a line's error is recorded here, -1, and the line ends " ok" */
 #define LINE_STATUS  (BUF0_W - 9)               /* how the last line ended: 0 QUIT, 1 completed, 2 an error */
-#define PORT         (BUF0_W - 10)              /* the node's port: a write to it blocks until its neighbour, the kernel, has served it */
+#define PORT         (BUF0_W - 32)              /* the node's ports (node.h): V4_PORTS of them, then "any port" and "which port".  Port 0 is where its requests go. */
 #define WORD_DEFINED   (BUF0_W - 11)             /* the xt of the word that is told of each new entry, or 0 */
 #define WORD_FORGOTTEN (BUF0_W - 12)             /* the xt of the word that is told when entries go, or 0 */
 #define LOG_LEVEL    (BUF0_W - 5)               /* the level at or below which a message is printed */