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.
