feat(v4.0.0): vocabularies, to FORTH-79

- VOCABULARY DEFINITIONS CONTEXT CURRENT FORTH, and v3's ORDER.  A name is
  looked up in the CONTEXT vocabulary and then in FORTH; a new entry goes
  into the CURRENT vocabulary; : makes the CURRENT vocabulary CONTEXT;
  FORTH is immediate.
- WORDS lists the CONTEXT vocabulary.  FORGET takes what was defined later
  out of every vocabulary, and a vocabulary that goes gives way to FORTH.
- v3's vocabularies separate nothing: a word defined in one is found from
  every other, and one redefined in a vocabulary replaces FORTH's for good.
- tests/test_host_quit.c: isolation, chaining to FORTH, two vocabularies
  with the same names, FORGET across them; and DUMP is now checked to put
  BASE back.

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

v4/tests/host_map.h

@@ -40,6 +40,9 @@
 #define FVARS        (TOP - 105)                /* (F): forth.v4, 7 cells */
 #define SVARS        (TOP - 540 - (v4_cell)V4_DATA_DEPTH)   /* (S): where PICK and ROLL set stack values aside, one cell for each cell of the stack */
 #define WVARS        (TOP - 99)                 /* (W): numout.v4, 3 cells */
+#define CONTEXT      (TOP - 64)                 /* the vocabulary names are looked up in */
+#define CURRENT      (TOP - 63)                 /* the vocabulary new words go into */
+#define VOC_LINK     (TOP - 50)                 /* the newest vocabulary */
 #define FENCE        (TOP - 26)                 /* FORGET's lower limit */
 #define HLD          (TOP - 25)                 /* where the pictured number has got to */
 #define QVARS        (TOP - 52)                 /* (Q): quit.v4, 2 cells */
@@ -133,6 +136,9 @@ static int host_load(v4_text *tx, v4_node *n, const char *const *files, unsigned
     v4_text_constant(tx, "MAX-INT", (v4_cell)(V4_MSB - 1u));
     v4_text_constant(tx, "HLD", HLD);
     v4_text_constant(tx, "FENCE", FENCE);
+    v4_text_constant(tx, "CONTEXT", CONTEXT);
+    v4_text_constant(tx, "CURRENT", CURRENT);
+    v4_text_constant(tx, "VOC-LINK", VOC_LINK);
     v4_text_constant(tx, "HEND", HEND);
     v4_text_constant(tx, "-HFLOOR", -(HEND - 62));
     v4_text_constant(tx, "DSTACK-DEPTH", DSTACK_REG);
@@ -147,6 +153,10 @@ static int host_load(v4_text *tx, v4_node *n, const char *const *files, unsigned
             return 0;
         }
     }
+    /* one vocabulary, FORTH, whose head cell is LATEST */
+    n->mem[CONTEXT] = LATEST;
+    n->mem[CURRENT] = LATEST;
+    n->mem[VOC_LINK] = 0;
     return 1;
 }