fix(core): search order is authoritative; host words validate stack args

- Dictionary::find no longer falls back to the newest entry across all
  wordlists when the search order has no match (Forth 2012 16.3.3;
  gforth and SwiftForth agree). Cross-engine corpus program guards it.
- ~40 argument-taking host words (RND-SEED, ACCEPT, RESIZE, ALLOCATE,
  SEARCH, SUBSTITUTE, ROLL, M*, UM/MOD, SF@/SF!/DF@/DF!, F./FE./FS./F~,
  2R@, ...) popped or read stack cells with no underflow check; on an
  empty stack the pointer silently drifted past its base. New host_need/
  host_fneed/host_fpop checked helpers; class-wide regression test
  drives every word on an empty stack.

crates/core/src/dictionary.rs

@@ -192,10 +192,9 @@ impl Dictionary {
                     }
                 }
             }
-            // Fallback: return newest entry across all wordlists
-            if let Some(&(_wid, word_addr, fn_index, is_immediate)) = entries.last() {
-                return Some((word_addr, WordId(fn_index), is_immediate));
-            }
+            // In no wordlist of the search order: not findable
+            // (Forth 2012 §16.3.3 — the order is authoritative).
+            return None;
         }
 
         // Fallback: linked-list walk (for words not yet in the index)