gym entrance mat lies flat; fence posts never ride a support

The mat inside every gym door ($06 over $16, 28 placements and nowhere
else) is drawn from straight above, but $06 had been swept into the wall
band -- so the mat's top half stood as a 16px box in the doorway, and the
cell rule could not save it because a pin outranks the cell. Both halves
are now pinned ground.

And the `post` pool no longer takes the support lift. A post stands IN
the ground by definition rather than on top of something, but its cell is
blocked like any other post, so the blocked-cell test that separates
furniture-top props from floor seats cannot tell it apart. Pinning the
cliff slope chain gave posts along a cliff edge an authored box to their
south and hoisted them onto the clifftop.

data/voxel_heights.lua

@@ -212,7 +212,7 @@ return {
       -- $0D/$0E and $1D/$1E), which is drawn 32px tall and so reads as
       -- two stacked courses of the same band.
       wall = { 34, 35, 50, 51,
-               5, 6, 16,
+               5, 16,
                15, 36, 37, 38, 39, 53, 62, 66,
                68, 69, 70, 71, 84, 86, 87,
                32, 33, 48, 49,
@@ -237,7 +237,12 @@ return {
       -- it is always the top half of a walkable FLOOR cell, so rule 3
       -- already lays it flat.  Lorelei's room uses the same pair for
       -- the paved plaza that stands out of her water.
-      ground = { 4 },
+      -- the entrance mat every gym lays inside its door ($06 over $16,
+      -- 28 placements and nowhere else): drawn from straight above, so
+      -- it lies FLAT.  $06 was in the wall band above, which stood the
+      -- mat's top half up as a 16px box in the doorway -- the cell rule
+      -- could not save it, because a pin outranks the cell.
+      ground = { 4, 6, 22 },
       -- Celadon's hedge: a round shrub, one per cell ($2C/$2D over
       -- $2E/$2F), 35 of them planted wall to wall.  Round drawings get
       -- the cylinder archetype -- one voxel ball per 16x16 cell carved