starkernel: item 4.3.4 -- checkpoint, draw a cube (no new bugs)

Adds VERT/EDGE/CUBE to capsules/fabric.4th (blocks 4913-4915). VERT
( n -- x y z ) reads bits 0/1/2 of a corner index as the X/Y/Z sign
(+-CS from center), so all 8 cube corners come from one word. EDGE
resolves both corners via VERT and calls LINE; CUBE is 12 EDGE calls
(4 bottom, 4 top, 4 vertical).

First item in the 4.3.3.x sequence with no new bug found -- a small signal
that Q.TO-INT, the VARIABLE alignment fix, and the LINE-STUCK? cap were
the real gaps rather than something still lurking in LINE/PROJECT/CART-Y.

Verified live on amd64: a centered, half-size-100 cube renders correctly
-- front/back face squares, back face offset diagonally up-right by
exactly the 45-degree cavalier projection's depth term, all 12 edges
connecting at the right corners.

All three architectures boot clean to ok> with the DoE completing;
dict_hash identical across all three and unchanged from 4.3.3a/4.3.3b.

FABRIC.md item 4.3.4 marked done. This is the checkpoint -- 4.3.x
groundwork stops here for review per this item's own acceptance criterion.

capsules/fabric.4th

@@ -150,3 +150,30 @@ Block 4912
     CZ @ CCOLOR @
     LINE
   LOOP ;
+
+Block 4913
+( CUBE: wireframe, 8 vertices via bit-coded corner index n  )
+( (bit0=x, bit1=y, bit2=z; set=+CS, clear=-CS from center). )
+VARIABLE CS
+: VERT ( n -- x y z )
+  DUP 1 AND IF CS @ ELSE CS @ NEGATE THEN CX @ +
+  SWAP DUP 2 AND IF CS @ ELSE CS @ NEGATE THEN CY @ +
+  SWAP DUP 4 AND IF CS @ ELSE CS @ NEGATE THEN CZ @ +
+  SWAP DROP ;
+
+Block 4914
+( EDGE: draws one cube edge between vertex indices n1,n2.   )
+VARIABLE EN2
+: EDGE ( n1 n2 color -- )
+  >R EN2 ! VERT EN2 @ VERT R> LINE ;
+
+Block 4915
+( CUBE: 12 edges -- 4 bottom, 4 top, 4 vertical.            )
+: CUBE ( cx cy cz s color -- )
+  CCOLOR ! CS ! CZ ! CY ! CX !
+  0 1 CCOLOR @ EDGE  1 3 CCOLOR @ EDGE
+  3 2 CCOLOR @ EDGE  2 0 CCOLOR @ EDGE
+  4 5 CCOLOR @ EDGE  5 7 CCOLOR @ EDGE
+  7 6 CCOLOR @ EDGE  6 4 CCOLOR @ EDGE
+  0 4 CCOLOR @ EDGE  1 5 CCOLOR @ EDGE
+  2 6 CCOLOR @ EDGE  3 7 CCOLOR @ EDGE ;