feat(v4.0.0): (CATCH) and (EMIT-HOOK), the two nucleus hooks POST needs

(CATCH): while it is not zero, a line that ends in an error sets it to -1
and ends " ok" instead of " ERROR".  (EMIT-HOOK): the xt of a word that is
given each character EMIT would send to the console.  The prompt loop sets
(EMIT-HOOK) to 0 at the end of every line.  docs/v4.0.0/NUCLEUS.md 6.3,
amended: it said one nucleus word would do.

Verified: make -C v4 test passes at both widths; by hand at the hosted
prompt, an unknown word and a division by zero are caught, the output hook
receives every character, and an uncaught error still says ERROR.

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

v4/capsule/quit.v4

@@ -35,14 +35,25 @@ macro R-CLEAR   RSTACK-DEPTH b! a !b   endmacro
 \ structures.  A definition that was under way stays hidden.
 : (RESET)   0 STATE a! !  (CG-RESET)  CFBASE (CFP) a! ! ;
 
+\ CATCHING A LINE'S ERROR.  While the variable (CATCH) is not zero, a line
+\ that ends in an error -- a fault, or a code in NODE-ERROR -- does not say
+\ ERROR: (CATCH) is set to -1 and the line ends " ok".  Whatever message the
+\ error printed has gone wherever EMIT was sending it.  POST uses this to
+\ go on to its next case, and to check the cases that must fail
+\ (docs/v4.0.0/NUCLEUS.md 6.3).  (EMIT-HOOK), core.v4, is set to 0 here at
+\ the end of every line, caught or not.
+\
 \ ( k -- )   the loop.  It is entered with what to say first -- 0 nothing,
 \ 1 " ok", anything else " ERROR" -- and never returns.  It is always jumped
 \ to, never called, by something that has emptied the return stack (or by a
 \ fault, which empties it).  Each line is then interpreted with one return
 \ entry under it, this word's call of INTERPRET.
 : (REPL)
+  (EMIT-HOOK) b! 0 !b                                                 \ the console is the console again
   if GO  -1 + if OK
-      drop  (RESET)  $52524520 (EMIT4) $524F (EMIT4) CR  jump READ    \ " ERROR"
+      drop  (RESET)
+      (CATCH) b! @b if LOUD  drop  -1 !b  1 jump (REPL)               \ caught: the line ends " ok"
+      LOUD: drop  $52524520 (EMIT4) $524F (EMIT4) CR  jump READ       \ " ERROR"
   OK: drop  $6B6F20 (EMIT4) CR  jump READ                             \ " ok"
   GO: drop
   READ:
@@ -57,6 +68,9 @@ macro R-CLEAR   RSTACK-DEPTH b! a !b   endmacro
 
 \ FORTH-79: clear the return stack, set execution mode, return control to
 \ the terminal; no message is given.  The data stack is left as it is.
+header (CATCH) inline       : (CATCH)'      (CATCH) ;
+header (EMIT-HOOK) inline   : (EMIT-HOOK)'  (EMIT-HOOK) ;
+
 header QUIT
 : QUIT    R-CLEAR  (RESET)  CR  0 jump (REPL)