Add FENCE word (SDK v1.9.0 scoping); fix severe pre-existing FORGET use-after-free

FENCE ( -- ) exposes the dict_fence_latest/dict_fence_here state FORGET
already honored internally, letting callers (e.g. a future SDK capsule)
raise the boundary after loading their own content -- no new VM fields,
no policy logic beyond exposing existing state.

Writing a direct test for it surfaced a real, severe, pre-existing bug in
FORGET's relink logic, unrelated to FENCE itself and reproducible with the
original boot-time fence alone:

- Forgetting the single newest word incorrectly destroyed every other word
  back to the fence too, not just the target.
- Forgetting an older word (correctly cascading to remove newer words too,
  per FORTH-79 semantics) crashed with SIGSEGV.

Root cause: the relink code's target_prev pointer was, by construction,
always inside the range the preceding loop had just freed whenever target
wasn't vm->latest -- so writing through it was a use-after-free every time
that branch executed. Fixed by removing the target_prev tracking and both
branches entirely; vm->latest unconditionally becomes target_next (target's
own captured, still-valid link) after the free loop, correct in every case.

Added a FENCE test suite to dictionary_manipulation_words_test.c (Module 14)
including the exact regression case (forgetting the newest word must not
disturb an older one). Verified zero warnings and identical POST/dict_hash
results across all three kernel architectures.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/test_runner/modules/dictionary_manipulation_words_test.c

@@ -70,6 +70,34 @@ static WordTestSuite dict_manip_word_suites[] = {
         3, {0}
     },
 
+    {
+        /* FENCE ( -- ): raises the FORGET boundary to the current dictionary
+         * top. Added alongside a real bug fix in FORGET itself, caught while
+         * building this word -- forgetting the single newest word used to
+         * incorrectly destroy every other word back to the fence too (a
+         * use-after-free in the relink logic, not just an off-by-one), a
+         * SIGSEGV in the worst case. "forget_latest_keeps_predecessor" below
+         * is exactly the regression case that fix addresses. */
+        "FENCE", {
+            {
+                "basic", "CREATE fnc1 FENCE CREATE fnc2 FORGET fnc2",
+                "Should forget a word defined after FENCE", TEST_NORMAL, 0, 1, {0}
+            },
+            {
+                "protects_older", "CREATE fnc3 CREATE fnc4 FENCE FORGET fnc3",
+                "Should refuse to forget a word defined before FENCE", TEST_ERROR_CASE, 1, 1, {0}
+            },
+            {
+                "forget_latest_keeps_predecessor",
+                "CREATE fnc5 FENCE CREATE fnc6 CREATE fnc7 FORGET fnc7 fnc6 DROP",
+                "Forgetting the newest word must not disturb an older one still above FENCE",
+                TEST_NORMAL, 0, 1, {0}
+            },
+            {NULL, NULL, NULL, TEST_NORMAL, 0, 0, {0}}
+        },
+        3, {0}
+    },
+
     {
         "IMMEDIATE", {
             {"basic", ": test3 42 ; IMMEDIATE test3 . CR", "Should execute immediately", TEST_NORMAL, 0, 1, {0}},

src/word_source/defining_words.c

@@ -76,6 +76,7 @@ static void defining_word_literal(VM * vm);
 static void defining_word_does(VM * vm);
 static void defining_word_immediate(VM * vm);
 static void dictionary_word_forget(VM * vm);
+static void dictionary_word_fence(VM * vm);
 
 /* ───────────────────────────── Runtimes ───────────────────────────── */
 
@@ -579,18 +580,16 @@ static void dictionary_word_forget(VM *vm) {
     int forget_failed = 0;
     sf_mutex_lock(&vm->dict_lock);
 
-    /* Find target and its previous entry */
-    DictEntry *prev = NULL, *e = vm->latest;
-    DictEntry *target = NULL, *target_prev = NULL;
+    /* Find target */
+    DictEntry *e = vm->latest;
+    DictEntry *target = NULL;
     while (e) {
         if (!(e->flags & WORD_HIDDEN) &&
             e->name_len == (uint8_t) nlen &&
             memcmp(e->name, namebuf, (size_t) nlen) == 0) {
             target = e;
-            target_prev = prev;
             break;
         }
-        prev = e;
         e = e->link;
     }
     if (!target) {
@@ -649,14 +648,17 @@ static void dictionary_word_forget(VM *vm) {
         e = next;
     }
 
-    /* Relink: if target_prev exists, point it past the freed chain */
-    if (target_prev) {
-        target_prev->link = target_next; /* Skip freed entries */
-        vm->latest = target_prev; /* Latest is now the entry before target */
-    } else {
-        /* No prev means target was latest - reset to fence */
-        vm->latest = vm->dict_fence_latest;
-    }
+    /* Relink: FORGET removes target and everything newer (closer to
+     * vm->latest) -- the free loop above just freed every entry from
+     * vm->latest through target inclusive, which by definition includes
+     * any entry between them. There is nothing left to "skip past" --
+     * target_next (target's own surviving link) is always the correct
+     * new head, whether or not target was vm->latest itself. (Previously
+     * this branched on whether an entry existed between vm->latest and
+     * target, and relinked through it -- but that entry was always inside
+     * the just-freed range, making it a use-after-free on every FORGET of
+     * a non-latest word.) */
+    vm->latest = target_next;
 
     vm->here = (size_t) new_here;
 
@@ -669,6 +671,21 @@ forget_exit:
         return;
 }
 
+/* FENCE ( -- )  raise the FORGET boundary to the current dictionary top.
+ * dict_fence_latest/dict_fence_here already exist and are already honored
+ * by FORGET (set once at bootstrap, protecting the base wordset); this is
+ * the first word exposing that state so callers can advance it further --
+ * e.g. an SDK capsule calling FENCE after loading protects its own
+ * definitions the same way the base wordset is already protected. */
+static void dictionary_word_fence(VM *vm) {
+    if (!vm) return;
+    sf_mutex_lock(&vm->dict_lock);
+    vm->dict_fence_latest = vm->latest;
+    vm->dict_fence_here = vm->here;
+    sf_mutex_unlock(&vm->dict_lock);
+    log_message(LOG_DEBUG, "FENCE: raised to HERE=%ld", (long) vm->dict_fence_here);
+}
+
 /* DOES> — IMMEDIATE: finalize the defining word’s create-part and compile DOES>-body */
 static void defining_word_does(VM *vm) {
     if (!vm) return;
@@ -755,6 +772,7 @@ void register_defining_words(VM *vm) {
 
     /* Dictionary management */
     register_word(vm, "FORGET", dictionary_word_forget);
+    register_word(vm, "FENCE", dictionary_word_fence);
 
     /* Compile helpers — immediate */
     register_word(vm, "COMPILE", defining_word_compile);