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/capsule/system.v4

@@ -102,8 +102,9 @@ header FORGET
   PRUNED: drop
   CONTEXT a! @ (Q) a! @ inv + 1 + -if C1  drop jump C2
   C1: drop  (LATEST) CONTEXT a! !
-  C2: CURRENT a! @ (Q) a! @ inv + 1 + -if C3  drop ;
-  C3: drop  (LATEST) CURRENT a! ! ;
+  C2: CURRENT a! @ (Q) a! @ inv + 1 + -if C3  drop jump TOLD
+  C3: drop  (LATEST) CURRENT a! !
+  TOLD: (Q) a! @  jump (FORGOTTEN)           \ the kernel's records of them go too (dict.v4)
   MISSING: drop  (UNKNOWN) drop ;
 
 \ ---- the system as a whole -------------------------------------------------------
@@ -142,6 +143,7 @@ header COLD
   (BOOT) a! @ 2/ 2/  FENCE a! !
   (LATEST) CONTEXT a! !  (LATEST) CURRENT a! !  0 VOC-LINK a! !
   10 BASE a! !  0 SCR a! !  2 (LOG-LEVEL) a! !  0 (ACL-HOOK) a! !  EMPTY-BUFFERS
+  (BOOT) a! @ 2/ 2/  (FORGOTTEN)             \ the kernel's records of what has gone go too (dict.v4)
   $54524F46 (EMIT4) $39372D48 (EMIT4) $6C6F4320 (EMIT4) $74532064 (EMIT4) $747261 (EMIT4) CR
   $74737953 (EMIT4) $69206D65 (EMIT4) $6974696E (EMIT4) $7A696C61 (EMIT4) $2E6465 (EMIT4) CR
   jump ABORT