feat(v4.0.0): address faults -- an address outside memory is guarded (D-14)

Ruled 2026-10-04: guarded, an error shown, back to the prompt.

- The executor checks every address a programme uses (P, A, B) before
  using it.  Outside memory the opcode does nothing, the rest of its word
  is not executed, and P becomes the node's fault handler; a node with no
  handler stops.  v4_node_load/store never index outside memory.
- capsule/quit.v4: (FAULT), the host node's handler, prints
  "Address out of range", ends an open definition, prints ERROR and
  returns to the prompt.
- tests: every memory opcode and P in test_exec.c; from the prompt, from
  inside nested words and loops, in test_host_quit.c.

This closes the hole node.h described: a wild address used to index the
model's own memory gigabytes out of bounds.

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

v4/tests/test_host_quit.c

@@ -20,6 +20,7 @@
  *     the text and carries on.  Compiled into a word, v3's crashes (SIGSEGV).
  *   - control words that do not match give ERROR alone; v3 names the word.
  *   - QUERY takes 80 characters (FORTH-79); v3's line is 255.
+ *   - an address outside memory says so (D-14); v3 gives ERROR alone.
  */
 #include "v4/text.h"
 #include "v4/testcode.h"
@@ -38,7 +39,7 @@ static v4_node       n;
 static v4_exec_state es;
 static v4_heat       h;
 static v4_text       tx;
-static v4_cell       w_quit, w_key, w_key_end, capsule_latest;
+static v4_cell       w_quit, w_key, w_key_end, w_fault, capsule_latest;
 static char          out[V4_CONSOLE_CAP + 1];
 static long          last_steps;
 
@@ -71,6 +72,7 @@ static void boot_with(unsigned depth)
     n.mem[NODE_ERROR] = 0;
     v4_node_console_attach(&n, CONSOLE_TX);
     v4_node_console_input_attach(&n, CONSOLE_RX, CONSOLE_ST);
+    v4_node_fault_attach(&n, w_fault);
     v4_dstack_reset(&n.ds);
     v4_rstack_reset(&n.rs);
     v4_exec_reset(&es);
@@ -180,6 +182,7 @@ int main(void)
     w_quit = v4_text_word(&tx, "QUIT");
     w_key = v4_text_word(&tx, "KEY");
     w_key_end = v4_text_word(&tx, "CR");               /* the word after KEY in core.v4 */
+    w_fault = v4_text_word(&tx, "(FAULT)");
     capsule_latest = v4_text_latest(&tx);
     CHECK(w_key_end > w_key && w_key_end - w_key <= 12, "KEY is the few words before CR");
 
@@ -230,6 +233,29 @@ int main(void)
     CHECK(is(say("67 EMIT\n"), "C ok\nok> ") && is(say("H3\n"), "UNKNOWN WORD: 'H3'\n ERROR\nok> "), "and the definition is gone");
     CHECK(is(say(": H4 68 EMIT ; H4\n"), "D ok\nok> "), "the next definition compiles and runs");
 
+    /* ---- an address outside memory (D-14): a message, ERROR, and the prompt ---- */
+    {
+        static const char *const lines[] = {
+            "65 EMIT -1 @ 66 EMIT\n", "65 EMIT 5 -1 ! 66 EMIT\n", "65 EMIT 16384 @ 66 EMIT\n", "65 EMIT 5 16384 ! 66 EMIT\n",
+            "65 EMIT 99999999 C@ 66 EMIT\n", "65 EMIT 7 -4 C! 66 EMIT\n", "65 EMIT -1 EXECUTE 66 EMIT\n", "65 EMIT 1 -1 +! 66 EMIT\n",
+            "65 EMIT PAD -8 4 CMOVE 66 EMIT\n", "65 EMIT -9 COUNT 66 EMIT\n", "65 EMIT -9 3 TYPE 66 EMIT\n",
+            ": F0 -1 @ ; : F1 F0 ; : F2 F1 ; : F3 65 EMIT F2 66 EMIT ; F3 67 EMIT\n",
+            ": G0 9 0 DO I 3 = IF 0 -1 ! THEN LOOP ; : G1 65 EMIT G0 66 EMIT ; G1\n",
+        };
+        boot();
+        for (i = 0; i < sizeof lines / sizeof lines[0]; i++) {
+            unsigned before = n.faults;
+            CHECK(is(say(lines[i]), "AAddress out of range\n ERROR\nok> "), "fault %u", i);
+            CHECK(n.faults == before + 1 && !n.stopped && n.mem[STATE] == 0, "fault %u: one fault, and the node is at its prompt", i);
+            CHECK(is(say("1 2 + 48 + EMIT\n"), "3 ok\nok> "), "and the next line runs, after fault %u", i);
+        }
+        CHECK(is(say("16383 @ DROP 0 @ DROP 67 EMIT\n"), "C ok\nok> "), "the first and last words of memory are not faults");
+        CHECK(is(say(": H5 1 IF [ -1 @ ]\n"), "Address out of range\n ERROR\nok> ") && n.mem[STATE] == 0 && n.mem[CFP] == CFS_W,
+              "a fault while a definition is open ends it");
+        CHECK(is(say("H5\n"), "UNKNOWN WORD: 'H5'\n ERROR\nok> ") && is(say(": H6 68 EMIT ; H6\n"), "D ok\nok> "), "and the next definition compiles and runs");
+        CHECK(v4_node_guards_intact(&n), "guards intact after the faults");
+    }
+
     /* ---- the text is in the definition, a counted string after the call ---- */
     boot();
     for (v4_cell k = n.mem[DP] / 4; k < DICT_END_W; k++) n.mem[k] = (v4_cell)-1;       /* memory that was not zero */