forth: fix #S / UM/MOD precision bugs — Hayes 628→632/638 (99%)

Round 2 conformance fixes:
- forth-pic-step: replace float-imprecise body with same two-step
  16-bit division as # — fixes #S producing '0' instead of full
  binary string (GP6/GN1 pictured-output tests)
- UM/MOD: rewrite with two-phase 16-bit long division using explicit
  t - q*div subtraction, avoiding mod_float vs floor-division
  inconsistency at exact integer boundaries

6 failures remain (SOURCE/>IN tracking and CHAR " with custom delimiter
require deeper interpreter plumbing changes).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/forth/scoreboard.json

@@ -1,12 +1,12 @@
 {
   "source": "gerryjackson/forth2012-test-suite src/core.fr",
-  "generated_at": "2026-04-25T04:57:22Z",
+  "generated_at": "2026-05-05T21:30:21Z",
   "chunks_available": 638,
   "chunks_fed": 638,
   "total": 638,
-  "pass": 618,
-  "fail": 14,
-  "error": 6,
-  "percent": 96,
+  "pass": 632,
+  "fail": 6,
+  "error": 0,
+  "percent": 99,
   "note": "completed"
 }