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/src/testcode.c

@@ -6,6 +6,7 @@ long v4_test_call(v4_node *n, v4_exec_state *es, v4_heat *h,
 {
     long words = 0;
 
+    n->stopped = 0;                             /* a stop left by an earlier call is over */
     v4_rstack_push(&n->rs, V4_TEST_HALT);
     n->p = entry;
     while (n->p != V4_TEST_HALT) {