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/input.v4

@@ -71,9 +71,17 @@ header QUERY
 \ (CONVERT and NUMBER use (P)+2 .. (P)+5; they and WORD never run inside
 \ each other.)
 
+\ ( -- c )   the character WORD is at.  A zero reads as a blank when words
+\ are being split at blanks: a block that has never been written is all
+\ zeros, and so is the end of one whose lines were put there one at a time.
+: (SRC@)
+  (P)+6 a! @ (SRC) a! @ + C@  if Z ;
+  Z: drop  (P) a! @ -32 + if B  drop 0 ;
+  B: drop 32 ;
+
 \ In line, not called: WORD may have only a few return entries to spare.
 macro (LEFT)   (P)+6 a! @  (P)+1 a! @  -            endmacro   \ ( -- d )  d < 0 while inside the text
-macro (CH)     (P)+6 a! @ (SRC) a! @ + C@  (P) a! @  xor   endmacro   \ ( -- x )  x = 0 at a delimiter
+macro (CH)     (SRC@)  (P) a! @  xor   endmacro   \ ( -- x )  x = 0 at a delimiter
 macro (STEP)   (P)+6 a! @ 1 + !                     endmacro
 
 \ ( c -- baddr )   FORTH-79: characters are taken from TIB until the