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.

FABRIC.md

@@ -3797,10 +3797,30 @@ document and committing that amendment as its own item.*
   > than tall, matching unequal radii), and the earlier diagonal `LINE` test.
   > `fb/amd64/geom-test-circle-fixed.png`, `fb/amd64/geom-test-full-wordset.png`.
 
-- [ ] **4.3.4 — Checkpoint: draw a cube.** First real exercise of the 4.3.3/4.3.3a/4.3.3b
+- [x] **4.3.4 — Checkpoint: draw a cube.** First real exercise of the 4.3.3/4.3.3a/4.3.3b
   coordinate/projection/geometry machinery — cube edges use `LINE`. Stop and review here
   before scoping the next 4.3.x item — not expected to be fast. *Refs:* §27.4.
 
+  > **Done, 2026-08-07.** Block 4913–4915. Vertices are bit-coded: `VERT ( n -- x y z )`
+  > reads bits 0/1/2 of `n` as the sign of the X/Y/Z offset from center (`±CS`), so all 8
+  > corners come from one word instead of 8 hand-written coordinate triples. `EDGE
+  > ( n1 n2 color -- )` resolves both corners via `VERT` and calls `LINE`. `CUBE
+  > ( cx cy cz s color -- )` is 12 `EDGE` calls — 4 bottom, 4 top, 4 vertical — grouped by
+  > face for readability, not because the grouping means anything to the machinery.
+  >
+  > No new bugs this item — first time in the 4.3.3.x sequence that's been true, which is
+  > itself a small signal that `Q.TO-INT`/the `VARIABLE` alignment fix/the `LINE-STUCK?`
+  > cap were the real gaps, not something still lurking in `LINE`/`PROJECT`/`CART-Y`.
+  >
+  > **Verified live on amd64**: `640 400 0 100 16777215 CUBE` (white, half-size 100,
+  > centered mid-screen) completed cleanly, no errors, no `LINE-STUCK?` trips. Screendump
+  > (`fb/amd64/cube-4.3.4.png`) shows a correct wireframe cube — front face square, back
+  > face square offset diagonally up-right by exactly the 45° cavalier projection's
+  > depth term, all 12 edges connecting at the right corners, no crossed or broken lines.
+  >
+  > **This is the checkpoint** — 4.3.x groundwork stops here for review per this item's own
+  > acceptance criterion, before scoping whatever comes next.
+
   ⋯ *(4.3.x is open-ended — more items get appended here as Console work is scoped item by*
   *item, developed on the fly per §25.0. 4.4 below is unaffected by anything added above*
   *this marker.)*

capsules/BLOCK_MAP.md

@@ -1,5 +1,5 @@
 # Capsule Block Manifest — Auto-generated
-<!-- Generated by mkcapsule --manifest  2026-08-07T19:19:40Z -->
+<!-- Generated by mkcapsule --manifest  2026-08-07T19:27:53Z -->
 <!-- DO NOT EDIT — re-run mkcapsule --manifest to refresh.  -->
 <!-- Hand-written justifications and immutability notes live  -->
 <!-- in MANIFEST.md alongside this auto-generated index.      -->
@@ -13,7 +13,7 @@
 | `common:msg.4th` | 4055 | `0xa99c5bcd3877f80e` |
 | `doe-campaign.4th` | 4060, 4061, 4062, 4063, 4064, 4065 | `0x3d4549142d91ec20` |
 | `doe.4th` | 2100, 2101, 2102, 2103, 2104, 2105, 2106, 2107 | `0xb6ecf5374e8ee77c` |
-| `fabric.4th` | 4900, 4901, 4902, 4903, 4904, 4905, 4906, 4907, 4908, 4909, 4910, 4911, 4912 | `0x355ccc154aac15b3` |
+| `fabric.4th` | 4900, 4901, 4902, 4903, 4904, 4905, 4906, 4907, 4908, 4909, 4910, 4911, 4912, 4913, 4914, 4915 | `0x339aa4c724b95879` |
 | `hermes:init.4th` | 4100, 4101, 4102, 4103, 4104, 4105, 4106, 4107, 4108, 4109, 4114, 4115, 4116, 4117, 4118, 4119, 4120, 4121, 4142, 4143, 4144, 4145, 4146, 4147, 4148, 4149, 4150, 4151, 4152, 4153, 4154, 4155, 4156, 4157, 4158, 4159, 4175, 4176 | `0x85c7b311d1e5bf97` |
 | `init-0.4th` | 2200, 2201 | `0xd0a9550baf786bb3` |
 | `init-1.4th` | 4406, 4415, 4425, 4435 | `0x63e251adb0a03613` |
@@ -225,19 +225,22 @@
 | 4842 | `init-l8-transition.4th` | `0xbcc1a81976f0a4c9` | ok |
 | 4851 | `artemis:init.4th` | `0xc9e92cd18f4c7b49` | ok |
 | 4852 | `artemis:init.4th` | `0xc9e92cd18f4c7b49` | ok |
-| 4900 | `fabric.4th` | `0x355ccc154aac15b3` | ok |
-| 4901 | `fabric.4th` | `0x355ccc154aac15b3` | ok |
-| 4902 | `fabric.4th` | `0x355ccc154aac15b3` | ok |
-| 4903 | `fabric.4th` | `0x355ccc154aac15b3` | ok |
-| 4904 | `fabric.4th` | `0x355ccc154aac15b3` | ok |
-| 4905 | `fabric.4th` | `0x355ccc154aac15b3` | ok |
-| 4906 | `fabric.4th` | `0x355ccc154aac15b3` | ok |
-| 4907 | `fabric.4th` | `0x355ccc154aac15b3` | ok |
-| 4908 | `fabric.4th` | `0x355ccc154aac15b3` | ok |
-| 4909 | `fabric.4th` | `0x355ccc154aac15b3` | ok |
-| 4910 | `fabric.4th` | `0x355ccc154aac15b3` | ok |
-| 4911 | `fabric.4th` | `0x355ccc154aac15b3` | ok |
-| 4912 | `fabric.4th` | `0x355ccc154aac15b3` | ok |
+| 4900 | `fabric.4th` | `0x339aa4c724b95879` | ok |
+| 4901 | `fabric.4th` | `0x339aa4c724b95879` | ok |
+| 4902 | `fabric.4th` | `0x339aa4c724b95879` | ok |
+| 4903 | `fabric.4th` | `0x339aa4c724b95879` | ok |
+| 4904 | `fabric.4th` | `0x339aa4c724b95879` | ok |
+| 4905 | `fabric.4th` | `0x339aa4c724b95879` | ok |
+| 4906 | `fabric.4th` | `0x339aa4c724b95879` | ok |
+| 4907 | `fabric.4th` | `0x339aa4c724b95879` | ok |
+| 4908 | `fabric.4th` | `0x339aa4c724b95879` | ok |
+| 4909 | `fabric.4th` | `0x339aa4c724b95879` | ok |
+| 4910 | `fabric.4th` | `0x339aa4c724b95879` | ok |
+| 4911 | `fabric.4th` | `0x339aa4c724b95879` | ok |
+| 4912 | `fabric.4th` | `0x339aa4c724b95879` | ok |
+| 4913 | `fabric.4th` | `0x339aa4c724b95879` | ok |
+| 4914 | `fabric.4th` | `0x339aa4c724b95879` | ok |
+| 4915 | `fabric.4th` | `0x339aa4c724b95879` | ok |
 
 ## Conflicts
 

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 ;