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/editor.fth

@@ -0,0 +1,68 @@
+\ editor.fth -- the block editor, in FORTH.
+\
+\ This file is FORTH source: the node compiles it itself, a line at a time,
+\ as if it were typed.  No line is longer than 79 characters.
+\
+\ A block is a screen of 16 lines of 64 characters.  SCR is the screen being
+\ edited.  The editor is a vocabulary, EDITOR: its words are ordinary FORTH
+\ words used at the ordinary prompt, so everything else is there as well.
+\
+\   n EDIT     edit screen n: list it and make EDITOR the CONTEXT vocabulary
+\   L          list the screen             N  B     the next, the one before
+\   n T        type line n
+\   n P text   put the rest of the input line on line n
+\   n E        erase line n                WIPE     blank the whole screen
+\   n D        delete line n; the lines below move up; it is held in PAD
+\   n S        spread: a blank line at n; the lines from n move down
+\   n H        hold line n in PAD          n R      replace line n from PAD
+\   n I        insert PAD's line at n
+\   DONE       write what has changed and go back to FORTH
+\   a b COPY   copy screen a to screen b           (in FORTH)
+\
+\ Every change is UPDATEd; nothing reaches storage until FLUSH or DONE, or
+\ until the buffer is wanted for another block.
+
+FORTH DEFINITIONS
+
+\ ( baddr -- )  64 characters; a zero shows as a blank, others as a dot
+: (.LINE)
+  64 0 DO  DUP I + C@
+    DUP 0= IF DROP 32 THEN  DUP 32 < OVER 126 > OR IF DROP 46 THEN  EMIT
+  LOOP DROP ;
+\ ( n -- baddr )  where line n of screen SCR is
+: (LINE)
+  DUP 0< OVER 15 > OR ABORT" Line out of range"  64 * SCR @ BLOCK + ;
+
+\ v3's three: a line shown, a line set from a string, the screen shown
+: L ( n -- )  (LINE) (.LINE) CR ;
+: S ( baddr u n -- )
+  (LINE) DUP 64 BLANK  SWAP 0 MAX 64 MIN CMOVE  UPDATE ;
+: SHOW ( -- )
+  ." Screen " SCR @ 0 .R ." :" CR
+  16 0 DO  I 2 .R ." : " I (LINE) (.LINE) CR  LOOP ;
+
+: COPY ( from to -- )  SWAP BLOCK SWAP BUFFER 1024 CMOVE UPDATE ;
+
+VOCABULARY EDITOR  EDITOR DEFINITIONS
+
+: T ( n -- )  DUP (LINE) SWAP 2 .R ." : " (.LINE) CR ;
+: L ( -- )  SHOW ;
+: E ( n -- )  (LINE) 64 BLANK UPDATE ;
+: P ( n -- )  (LINE) DUP 64 BLANK  0 WORD COUNT 64 MIN ROT SWAP CMOVE UPDATE ;
+: H ( n -- )  (LINE) PAD 64 CMOVE ;
+: R ( n -- )  PAD SWAP (LINE) 64 CMOVE UPDATE ;
+: D ( n -- )
+  DUP H  DUP 15 < IF
+    DUP 1+ (LINE) OVER (LINE) ROT 15 SWAP - 64 * CMOVE
+  ELSE DROP THEN  15 E ;
+: S ( n -- )
+  DUP 15 < IF  DUP (LINE) OVER 1+ (LINE) 3 PICK 15 SWAP - 64 * CMOVE>  THEN E ;
+: I ( n -- )  DUP S R ;
+: WIPE ( -- )  SCR @ BLOCK 1024 BLANK UPDATE ;
+: N ( -- )  1 SCR +! L ;
+: B ( -- )  -1 SCR +! L ;
+: DONE ( -- )  FLUSH [COMPILE] FORTH ;
+
+FORTH DEFINITIONS
+
+: EDIT ( n -- )  DUP BLOCK DROP  SCR !  EDITOR [ EDITOR ] L [ FORTH ] ;