feat(v4.0.0): a node asks its kernel by a blocking write to its port

ENGINE.md step 2, the carrier.  Ruled 2026-10-05 (V3-PARITY.md 1i), on
DECOMPOSITION.md section 6: a write to a port blocks until the neighbour
reads.

- node: v4_node_port_attach, v4_node_port_served; a store to the port keeps
  the value as the request and blocks the node
- exec: a blocked node executes nothing; served, it goes on from the opcode
  after the store, in the same instruction word; a fault meanwhile abandons
  the rest of the word
- compile.v4: n KERNEL-WORD name makes a word whose body writes n to the
  port; its arguments and results are on the data stack
- boot: the kernel's words are made by handing the node text, and requests
  are served between the node's opcodes; one no one serves is error 12
- BYE, the first kernel word: hosted it leaves the program, as hosted v3;
  on the lone node it is v3's cold restart
- ENGINE.md 3a: multiuser, multitasking, preemptive and cooperative, and
  what that asks of the engine

Verified: make -C v4 test passes at both widths, with tests/test_port.c;
hosted-check passes on three ISAs; clean qemu with STARFORTH_V4=1 on amd64,
aarch64 and riscv64 passes POST with the same hashes as hosted, and a
kernel word no one serves and BYE typed at each prompt are answered
(logs/20261005-185506, -185734, -190101; -185234 is an amd64 run in which
those two lines were not typed).

Not done: v3's own C functions serving a node.

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

v4/src/node.c

@@ -17,6 +17,7 @@ void v4_node_reset(v4_node *n)
     v4_node_fault_attach(n, -1);
     v4_node_stack_regs_attach(n, -1, -1);
     v4_node_error_attach(n, -1);
+    v4_node_port_attach(n, -1);
     v4_node_storage_attach(n, -1, 0, 0);
 }
 
@@ -119,6 +120,9 @@ void v4_node_store(v4_node *n, v4_cell addr, v4_cell value)
             n->console_dropped++;
         return;
     }
+    /* The port: the node keeps what it wrote and is blocked until its
+     * neighbour has taken it.  -1 when no port is attached. */
+    if (addr == n->port) { n->request = value; n->asking = 1; return; }
     /* The stack registers: a store empties the stack.  -1 when not attached. */
     if (addr == n->dstack_reg) { v4_dstack_clear(&n->ds); return; }
     if (addr == n->rstack_reg) { v4_rstack_clear(&n->rs); return; }
@@ -177,3 +181,15 @@ void v4_node_console_attach(v4_node *n, v4_cell addr)
     n->console_len = 0;
     n->console_dropped = 0;
 }
+
+void v4_node_port_attach(v4_node *n, v4_cell addr)
+{
+    n->port = addr;
+    n->asking = 0;
+    n->request = 0;
+}
+
+void v4_node_port_served(v4_node *n)
+{
+    n->asking = 0;
+}