Depth: a ball behind the near pillars and in front of the far ones

The demo for what V5 added. Four pillars at four distances, each ONE 8 by
8 tile stretched to its own width and height, and a ball walking past all
of them at a distance between two.

What it shows is the thing an ordering cannot. The ball is sprite NOUGHT
and every pillar is numbered after it, so table order puts the ball in
front of all four - and it is still hidden behind two of them, because the
depth buffer is asked per column. Caught mid-straddle in the checks: the
ball is 48 wide and the pillar 32, so it shows on both sides and nowhere
across the middle.

Writing it found the conceptual trap in the feature, which is now written
down where somebody will hit it. The pillars first carried their own
distance in their entries AND wrote that same distance into their columns,
so each was asked whether it was in front of itself - and 20 is not nearer
than 20, so all four vanished. THE BUFFER IS WHAT HAS BEEN DRAWN AND A
SPRITE'S DEPTH IS A QUESTION ASKED OF IT. Scenery writes it; it does not
ask.

Also found that a scheme only gives a colour to index one. The default
palette sets each scheme's paper and ink and nothing between them, so
art drawn in index two comes out black until a program writes a palette.

And the fixture disk's root directory was full: four blocks, 32 entries,
all taken, so adding an app failed the whole disk build. Loudly, which is
the right way round - but it is a wall that moves for free, so it is eight
blocks and 64 entries now.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW

Tests/expected/cosmosCrossDisk.out

@@ -24,6 +24,7 @@ Press.sbx               872
 Mode.sbx                48
 Flip.sbx                173
 Sprite.sbx              442
+Depth.sbx               672
 Crash.sbx               632
 vars.script             50
 blocks.script           343
@@ -40,7 +41,7 @@ outer.script            376
 inner.script            44
 loop.script             35
 crossed.txt             560
-27 files, 1 directory
+28 files, 1 directory
 > exit
 halted
 Execution halted.