feat(v4.0.0): the editor, redone in FORTH; COLD WARM PAGE VERSION DEFER

- capsule/editor.fth: the block editor as FORTH source, which the node
  compiles itself.  It is a vocabulary, EDITOR, used at the ordinary
  prompt: n EDIT, then L N B T P E D S H R I WIPE DONE.  v3's EDIT, a
  shell of its own, is not carried over (ruled 2026-10-05); v3's L S and
  SHOW are kept in FORTH as they were, with COPY.
- system.v4: COLD (the system as the loader left it), WARM, PAGE,
  VERSION, 79-STANDARD (FORTH-79: silent), and DEFER IS DEFER@ as v3.
- input.v4: where words are split at blanks a zero byte reads as a blank,
  so a block never written, or filled a line at a time, loads cleanly.
- tests/test_host_quit.c: the editor's source fed to the prompt line by
  line, every command on empty and full screens, what reaches storage;
  the system words; deferred words.

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

v4/capsule/system.v4

@@ -7,6 +7,7 @@
 \   FENCE   word address of the variable: FORGET will not remove an entry
 \           that starts below the address it holds
 \   VOC-LINK   word address of the variable: the newest vocabulary, or 0
+\   (BOOT)  word address of two cells: DP and (LATEST) as the loader left them
 
 header FENCE inline     : FENCE'  FENCE ;
 
@@ -104,3 +105,64 @@ header FORGET
   C2: CURRENT a! @ (Q) a! @ inv + 1 + -if C3  drop ;
   C3: drop  (LATEST) CURRENT a! ! ;
   MISSING: drop  (UNKNOWN) drop ;
+
+\ ---- the system as a whole -------------------------------------------------------
+\ (BOOT) is two cells the loader fills when the capsule is in place: DP and
+\ the newest FORTH entry as they then are.  COLD goes back to them.
+
+\ FORTH-79: execute to be sure a FORTH-79 system is there.  It is: nothing
+\ happens.  (v3's prints two lines and leaves a flag.)
+header 79-STANDARD
+: 79-STANDARD   ;
+
+\ ( -- )   which system this is, and its cell width
+header VERSION
+: VERSION
+  $72617453 (EMIT4) $74726F46 (EMIT4) $34762068 (EMIT4) $302E302E (EMIT4) $38314620 (EMIT4) $20 (EMIT4)
+  N-1 1 + 0 U-DOT-R  $7469622D (EMIT4) CR ;
+
+\ ( -- )   clear the screen and put the cursor at the top, as v3: ESC [2J ESC [H
+header PAGE
+: PAGE   27 EMIT 91 EMIT 50 EMIT 74 EMIT  27 EMIT 91 EMIT 72 jump EMIT
+
+\ ( -- )   as v3's message, and then ABORT: both stacks emptied, the prompt
+header WARM
+: WARM
+  $54524F46 (EMIT4) $39372D48 (EMIT4) $72615720 (EMIT4) $7453206D (EMIT4) $747261 (EMIT4) CR
+  $74737953 (EMIT4) $72206D65 (EMIT4) $61747365 (EMIT4) $64657472 (EMIT4) $2E (EMIT4) CR
+  jump ABORT
+
+\ ( -- )   the system as it was when the capsule had just been loaded:
+\ everything defined since is gone, there is one vocabulary, numbers are
+\ decimal, the block buffers are empty.  Then as WARM.
+header COLD
+: COLD
+  (BOOT) a! @  DP a! !
+  (BOOT)+1 a! @  (LATEST) a! !
+  (BOOT) a! @ 2/ 2/  FENCE a! !
+  (LATEST) CONTEXT a! !  (LATEST) CURRENT a! !  0 VOC-LINK a! !
+  10 BASE a! !  0 SCR a! !  EMPTY-BUFFERS
+  $54524F46 (EMIT4) $39372D48 (EMIT4) $6C6F4320 (EMIT4) $74532064 (EMIT4) $747261 (EMIT4) CR
+  $74737953 (EMIT4) $69206D65 (EMIT4) $6974696E (EMIT4) $7A696C61 (EMIT4) $2E6465 (EMIT4) CR
+  jump ABORT
+
+\ ---- deferred words (as v3) -------------------------------------------------------
+\ DEFER xxx   makes xxx, which does whatever word it has been given:
+\ ' yyy IS xxx   gives it one; DEFER@ xxx leaves the one it has.  A deferred
+\ word with none is an error (code 15, "Deferred word not set").
+: (DODEFER)
+  pop a! @ if UNSET  push ;                  \ go to it: it returns to xxx's caller
+  UNSET: drop  NODE-ERROR b! 15 !b ;
+header DEFER
+: DEFER   &(DODEFER) (C)+3 a! !  (DATA)  (C)+8 a! @ if NONE  drop 0 jump ,   NONE: drop ;
+: (is!)     a! ! ;
+: (defer@)  a! @ ;
+header IS immediate
+: IS
+  (')  STATE a! @ if NOW  drop (LIT,)  &(is!) jump (CALL,)
+  NOW: drop  jump (is!)
+header DEFER@ immediate
+: DEFER@
+  (')  STATE a! @ if NOW  drop (LIT,)  &(defer@) jump (CALL,)
+  NOW: drop  jump (defer@)
+