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/tests/host_map.h

@@ -29,13 +29,16 @@
 #define LATEST       (TOP - 12)
 #define STATE        (TOP - 13)
 #define CFP          (TOP - 14)                 /* the control-flow stack's pointer */
+#define DSTACK_REG   (TOP - 15)                 /* DSTACK-DEPTH (D-16) */
+#define RSTACK_REG   (TOP - 16)                 /* RSTACK-DEPTH */
 
 /* each capsule file's scratch cells */
 #define PVARS        (TOP - 24)                 /* (P): input.v4, 8 cells */
 #define DVARS        (TOP - 32)                 /* (D): dict.v4, 5 cells */
 #define CGVARS       (TOP - 48)                 /* (CG): codegen.v4, 14 cells */
 #define CVARS        (TOP - 62)                 /* (C): compile.v4, 10 cells */
-#define FVARS        (TOP - 27)                 /* (F): forth.v4, 3 cells */
+#define FVARS        (TOP - 105)                /* (F): forth.v4, 7 cells */
+#define SVARS        (TOP - 180)                /* (S): where PICK and ROLL set stack values aside, 10 cells */
 #define QVARS        (TOP - 52)                 /* (Q): quit.v4, 2 cells */
 
 /* buffers (word addresses; a byte address is four times this) */
@@ -103,6 +106,10 @@ static int host_load(v4_text *tx, v4_node *n, const char *const *files, unsigned
     v4_text_constant(tx, "(CFP)", CFP);
     v4_text_constant(tx, "(Q)", QVARS);
     v4_text_constant(tx, "(F)", FVARS);
+    v4_text_constant(tx, "(S)", SVARS);
+    v4_text_constant(tx, "DSTACK-DEPTH", DSTACK_REG);
+    v4_text_constant(tx, "RSTACK-DEPTH", RSTACK_REG);
+    v4_node_stack_regs_attach(n, DSTACK_REG, RSTACK_REG);
     v4_text_constant(tx, "CFBASE", CFS_W);
     v4_text_constant(tx, "CFEND", CFS_W + CFS_CELLS);
     for (i = 0; i < count; i++) {