feat(v4.0.0): division by zero is guarded (D-15); division words in the capsule

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

- capsule/forth.v4: / MOD /MOD */ */MOD M/MOD, M* and S>D join the
  vocabulary.  Each dividing word tests its divisor first; zero prints
  v3's message ("/: Division by zero"), ends an open definition, prints
  ERROR and returns to the prompt from however deep.
- SM/REM here has UM/MOD written into it and keeps its signs in memory,
  and */MOD has M* written in, so that division can be used inside words
  that other words call: / from four words deep, */MOD from three.
- forth.v4 now loads after quit.v4, which the guard leaves through.
- tests: v3 transcripts for each message; /MOD, /, MOD and */MOD against
  C on every pair of edge values.

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

v4/tests/host_map.h

@@ -35,6 +35,7 @@
 #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 QVARS        (TOP - 52)                 /* (Q): quit.v4, 2 cells */
 
 /* buffers (word addresses; a byte address is four times this) */
@@ -101,6 +102,7 @@ static int host_load(v4_text *tx, v4_node *n, const char *const *files, unsigned
     v4_text_constant(tx, "(C)", CVARS);
     v4_text_constant(tx, "(CFP)", CFP);
     v4_text_constant(tx, "(Q)", QVARS);
+    v4_text_constant(tx, "(F)", FVARS);
     v4_text_constant(tx, "CFBASE", CFS_W);
     v4_text_constant(tx, "CFEND", CFS_W + CFS_CELLS);
     for (i = 0; i < count; i++) {