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- DECOMPOSITION.md section 6: four named ports withdrawn for V4_PORTS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/test_port.c

@@ -12,7 +12,7 @@ static int failures = 0, checks = 0;
 #define CHECK(c,...) do{checks++; if(!(c)){failures++; printf("FAIL %s:%d: ",__FILE__,__LINE__); printf(__VA_ARGS__); printf("\n");}}while(0)
 
 /* The memory map is open (D-4); the test chooses the address. */
-#define PORT ((v4_cell)(V4_NODE_WORDS - 9u))
+#define PORT ((v4_cell)(V4_NODE_WORDS - 16u))
 #define MARK ((v4_cell)0x1234)
 #define HOME ((v4_cell)8)                /* where a word returns to when the test called it */