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/tools/hosted.c

@@ -31,6 +31,20 @@ static int console_key(void)
     return c == EOF ? V4_BOOT_KEY_END : c;
 }
 
+/* THE KERNEL'S WORDS, for a node that is a Linux program: the words that
+ * are C in hosted v3 because only the host can do them. */
+static const char *const kernel_words[] = { "BYE" };
+static int leaving;
+
+static void serve(v4_node *node, unsigned request)
+{
+    (void)node;
+    if (request == 1) {                              /* BYE ( -- ): as hosted v3, say so and leave */
+        fputs("Goodbye!\n", stdout);
+        leaving = 1;
+    }
+}
+
 /* The prompt is the host's, as it is the kernel's REPL's for a v3 VM: read a
  * line, hand it to the node, say how it ended. */
 int main(void)
@@ -40,6 +54,7 @@ int main(void)
 
     boot.n = &n; boot.es = &es; boot.h = &h; boot.im = &v4_capsule_image;
     boot.out = console_out; boot.key = console_key;
+    boot.words = kernel_words; boot.word_count = sizeof kernel_words / sizeof kernel_words[0]; boot.serve = serve;
     if (!v4_boot_run(&boot, disk, BLOCKS)) { fflush(stdout); return 1; }
 
     for (;;) {
@@ -53,6 +68,7 @@ int main(void)
         if (len && line[len - 1] == '\n') len--;
         how = v4_boot_line(&boot, line, (unsigned)len);
         if (how == V4_LINE_COMPLETED) fputs(" ok\n", stdout);
+        if (leaving) { fflush(stdout); return 0; }
         else if (how == V4_LINE_ERROR) fputs(" ERROR\n", stdout);
         else if (how == V4_BOOT_LINE_TOO_LONG) fputs("Line too long\n ERROR\n", stdout);
         else if (how == V4_BOOT_LINE_NO_INPUT) { putchar('\n'); return 0; }