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/core.v4

@@ -5,7 +5,7 @@
 \ written here as text for the text assembler (v4/include/v4/text.h).
 \
 \ Constants the loader supplies:  N-1 (cell bits - 1), NODE-ERROR, CONSOLE-TX,
-\ CONSOLE-RX, CONSOLE-STATUS, BASE.
+\ CONSOLE-RX, CONSOLE-STATUS, BASE, (EMIT-HOOK).
 \
 \ ERRORS (D-18).  A word that finds an error takes its arguments off the stack
 \ and stores a code in NODE-ERROR.  On a node with a prompt that store is a
@@ -91,8 +91,16 @@ header CMOVE
   DONE: drop drop drop ;
 
 \ ---- section 5.10: the console -------------------------------------------
+\ EMIT sends the character to the console, unless (EMIT-HOOK) holds the xt
+\ of a word: then that word is given it instead ( c -- ), and returns to
+\ EMIT's caller.  Everything the node prints goes through EMIT, so a hook
+\ sees all of it.  The prompt takes the hook off at the end of every line
+\ (quit.v4), so that nothing can leave the console silent.  POST uses it
+\ to compare what a case prints (docs/v4.0.0/NUCLEUS.md 6.3).
 header EMIT
-: EMIT ( c -- )        CONSOLE-TX b! !b ;
+: EMIT ( c -- )
+  (EMIT-HOOK) b! @b  if NONE  push ;
+  NONE: drop  CONSOLE-TX b! !b ;
 header KEY
 : KEY  ( -- c )
   L: CONSOLE-STATUS b! @b  if WAIT  drop  CONSOLE-RX b! @b ;