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/tests/host_map.h

@@ -68,6 +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 LOG_LEVEL    (BUF0_W - 5)               /* the level at or below which a message is printed */
 #define BOOT_CELLS   (BVARS + 14)               /* (BOOT): DP and LATEST as the loader left them, 2 cells */
 #define BUF0_W       (BVARS - 2 * 256)          /* the two block buffers, 256 cells each */
@@ -144,6 +145,7 @@ static int host_load(v4_text *tx, v4_node *n, const char *const *files, unsigned
     v4_text_constant(tx, "(EMIT-HOOK)", EMIT_HOOK);
     v4_text_constant(tx, "(CATCH)", CATCH);
     v4_text_constant(tx, "(LINE-STATUS)", LINE_STATUS);
+    v4_text_constant(tx, "(PORT)", PORT);
     v4_text_constant(tx, "BLOCK-NUMBER", STORAGE_REG);
     v4_text_constant(tx, "BLOCK-ADDRESS", STORAGE_REG + 1);
     v4_text_constant(tx, "BLOCK-COMMAND", STORAGE_REG + 2);