feat(v4.0.0): the stacks are guarded (D-16); DEPTH, PICK and ROLL

Ruled 2026-10-04, revising D-2: stack overflow and underflow are errors
that are shown and return to the prompt, not silent wrap-around.

- Each stack counts what it holds.  Before every opcode the executor
  checks that the stacks hold what it takes and have room for what it
  leaves; otherwise the opcode does nothing and the node faults, as for a
  bad address, to that kind's handler.  Every fault empties both stacks.
- The fault handler is now a table of five jumps: address, data overflow,
  data underflow, return overflow, return underflow.  The host node says
  "Stack overflow", "Stack underflow", "Return stack overflow",
  "Return stack underflow", then ERROR and the prompt.
- Two registers, DSTACK-DEPTH and RSTACK-DEPTH: a fetch reads the depth,
  a store empties the stack.  QUIT, ABORT and the error exits empty the
  return stack before they call anything; ABORT empties the data stack.
- capsule/forth.v4: DEPTH, PICK and ROLL, to FORTH-79 (counting from
  one).  PICK and ROLL set the values above the one wanted aside in
  memory, and work with the stack full.
- Division by zero now takes its operands off the stack, as v3 does.
- A colon with no room for its entry abandons the line.
- tests: every opcode at every depth of both stacks; the faults, the
  registers and the three words from the prompt.

The sizes are unchanged: ten values, nine return entries.

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

v4/capsule/compile.v4

@@ -17,8 +17,8 @@
 \   compile-only  it may not be executed by the interpreter.
 \ An immediate word is executed even when compiling.
 \
-\ THE STACKS ARE SHORT (D-2): ten cells of data, nine return entries, and no
-\ way to see how full they are.  The interpreter and compiler run on the same
+\ THE STACKS ARE SHORT (D-2): ten cells of data, nine return entries, and one
+\ more of either is a fault (D-16).  The interpreter and compiler run on the same
 \ stacks as the user's words, with the user's values beneath them, so:
 \   - every word here keeps what it works on in (C), and has at most three
 \     cells of its own on the data stack at any moment;
@@ -184,7 +184,7 @@ header :
   32 WORD (HEADER)
   LATEST (C)+2 a! @ xor if NONE  drop
   HIDDEN  (CG-RESET)  CFBASE (CFP) a! !  jump RIGHT-BRACKET
-  NONE: drop ;
+  NONE: drop  jump (ABANDON)                 \ no entry: the rest of the line is not a definition
 
 \ FORTH-79: compile the return, let the word be found, stop compiling
 header ; immediate compile-only