feat(v4.0.0): POST passes and is part of the boot; U* and U/MOD

The boot is now nucleus, forth79.4th, POST, prompt, on both products.

- forth79.4th: U* and U/MOD, the capsule's first colon definitions.  They
  are in the FORTH-79 Required Word Set and neither v3 nor v4 had them.
- post79.4th: 550 cases, 126 of the 130 required words.  443 are v3's with
  v3's result.  The rest follow three rulings (2026-10-05): address-
  dependent cases are checked for count, not value; where v3 departs from
  FORTH-79 the standard's result is expected; words v3 has no case for get
  cases written by hand.  v4/tools/post79_rules.py holds each exception
  with its reason and docs/v4.0.0/POST79.md lists them all.
- every case starts from an empty stack, DECIMAL and FORTH DEFINITIONS
- the boot requires POST's tally line with fail=0

Verified: tests=550 pass=550 fail=0 and identical PARITY lines on hosted
amd64, aarch64 and riscv64 (make -C v4 hosted-check) and on bare metal,
clean qemu with STARFORTH_V4=1, on the same three (logs/20261005-1619xx,
-1621xx, -1625xx).  A U/MOD broken on purpose fails five cases and stops
the boot.  make -C v4 test passes.

Not shown: all words but those two are still assembled, so POST has so far
tested the assembled words.  Nothing was typed at a bare-metal prompt.
Open: PAD 42 OVER ! faults on v4 (D-1).

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

capsules/v4/forth79.4th

@@ -3,4 +3,26 @@ Block 6000
 ( as colon definitions, loaded when the system boots.          )
 ( docs/v4.0.0/NUCLEUS.md.  Blocks 6000 up.                     )
 ( A word moves here from the assembled nucleus, v4/capsule,    )
-( once its colon definition passes POST.  None has moved yet.  )
+( once its colon definition passes POST.                       )
+Block 6001
+( U*  U/MOD -- FORTH-79 unsigned multiply and divide.          )
+( Neither v3 nor the v4 nucleus had them.                      )
+( The first two words count the bits in a cell.               )
+: (BITS) ( -- n )
+  0 1 BEGIN DUP WHILE 2* SWAP 1+ SWAP REPEAT DROP ;
+(BITS) CONSTANT (NB)
+VARIABLE (UD)
+( u1 u2 -- ud   the unsigned double product )
+: U* UM* ;
+Block 6002
+( ud u1 -- u2 u3   unsigned: u2 the remainder, u3 the quotient )
+( One bit a step: shift ud left, and take u1 from its high     )
+( cell whenever it goes, counting that in the low cell.        )
+( A zero divisor is error 11, Division by zero.                )
+: U/MOD
+  DUP 0= IF DROP DROP DROP 11 NODE-ERROR ! THEN  (UD) !
+  (NB) 0 DO
+    DUP 0< >R  2* OVER 0< IF 1+ THEN  SWAP 2* SWAP
+    R> IF (UD) @ - SWAP 1+ SWAP
+    ELSE DUP (UD) @ U< 0= IF (UD) @ - SWAP 1+ SWAP THEN THEN
+  LOOP SWAP ;