feat(v4.0.0): the node tells its kernel of its words; the boot seals the system

ENGINE.md 3b, the node's side of ruling A (a word's code is the node's, its
accounts the kernel's).

- dict.v4, system.v4: (WORD-DEFINED) ( xt -- ) is run when an entry is
  made, (WORD-FORGOTTEN) ( w -- ) when FORGET or COLD removes entries; with
  0 there no one is told, as on the hosted product
- test_host_quit.c: a kernel that keeps the list of words and is checked to
  hold exactly the node's dictionary after definitions, a vocabulary, an
  abandoned definition, FORGET, a refused FORGET and COLD; KERNEL-WORD
  called from the prompt and from a definition

Fixed, found while writing that test: since the capsules moved from build
time to boot time (294e6946), what COLD returns to and FORGET protects was
still the nucleus alone, so COLD lost U*, U/MOD and BYE and FORGET U* was
allowed.  The boot now seals the system when it has loaded it
(v4_image_seal), and hosted-check checks COLD, the capsule word after it,
the refused FORGET and BYE.

Verified: make -C v4 test passes at both widths (1283 checks in
test_host_quit.c); hosted-check passes on three ISAs; clean qemu with
STARFORTH_V4=1 on amd64, aarch64 and riscv64 passes POST, and COLD, U*
after it, FORGET U* (refused), an unserved kernel word and BYE typed at
each prompt are answered correctly (logs/20261005-193045, -193307, -193636).

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

v4/tests/host_map.h

@@ -69,6 +69,8 @@
 #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 WORD_DEFINED   (BUF0_W - 11)             /* the xt of the word that is told of each new entry, or 0 */
+#define WORD_FORGOTTEN (BUF0_W - 12)             /* the xt of the word that is told when entries go, or 0 */
 #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 */
@@ -146,6 +148,8 @@ static int host_load(v4_text *tx, v4_node *n, const char *const *files, unsigned
     v4_text_constant(tx, "(CATCH)", CATCH);
     v4_text_constant(tx, "(LINE-STATUS)", LINE_STATUS);
     v4_text_constant(tx, "(PORT)", PORT);
+    v4_text_constant(tx, "(WORD-DEFINED)", WORD_DEFINED);
+    v4_text_constant(tx, "(WORD-FORGOTTEN)", WORD_FORGOTTEN);
     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);