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/capsule/compile.v4

@@ -246,6 +246,20 @@ header ; immediate compile-only
   LATEST (FLAGS) a! @ -3 and !
   jump LEFT-BRACKET
 
+\ ( n -- )   KERNEL-WORD xxx   A WORD THAT ASKS THE KERNEL (docs/v4.0.0/
+\ ENGINE.md 3.3).  xxx is made a word whose whole body is "write n to the
+\ port": n is the number of a request, the port is the node's way to its
+\ kernel, and the write blocks the node until the kernel has served it
+\ (DECOMPOSITION.md section 6).  xxx's arguments are whatever it finds on
+\ the data stack and its results are what the kernel leaves there.  This is
+\ what a word that is a C function in v3 -- BIRTH, BLOCK, BYE -- is on a
+\ node.  Not FORTH-79 and not in v3, where such a word is registered from C.
+\ The constant (PORT) is the port's word address.
+header KERNEL-WORD
+: KERNEL-WORD
+  COLON  (LIT,)  (PORT) (LIT,)  30 (OP,)  14 (OP,)      \ n  port  b!  !b
+  jump SEMICOLON
+
 \ FORTH-79: return from the definition at this point
 header EXIT immediate compile-only
 : EXIT        0 jump (OP,)