docs(showcase): record the mirror-symmetry and wrapper-seating conventions

Three conventions every future piece inherits, each from a defect the
hay-bale pass found that no existing budget could see.

Even shaping terms and mirror symmetry (19): where a prop has mirrored
members, every closed-form term in the host's shaping function must be
even in the mirrored axis. hay-bale used sin(x*24), which is odd, so its
two belt stations sampled different surface heights and the wraps
shipped 5.4 mm out of step. Pair the mirrored shells and compare.

Wrappers follow the host's profile, and the host is finished first
(16/18): build, bevel and ground the host, then hang the wrapper on the
finished surface by raycasting the host's own cross-section. A constant
rectangle on a rounded host stands proud at the middle of each face and
is swallowed at the corners. Assert per wrapper shell that the loop
passes under the host, because the AABB zmin gate stays green when the
host grounds the box.

Seat depth is measured station-locally, along the vertex's own direction
from the station axis, never by nearest-surface distance: a wrapper in a
concave waist has its nearest host face on the bulge shoulder at a
neighbouring station. Bin by rounding to the nearest station, never by
flooring, or a station on a bin boundary splits its inner and outer
rings and the inner-only bin reports the outer offset as the seat depth.

Also sharpens the z-fight budget: count coplanar pairs cross-shell, not
merely over faces sharing no vertex. Two quads two steps apart on one
flat cap share no vertex and are coplanar by construction -- that read
96 pairs on hay-bale, none of them a hazard, which is the same
unsatisfiable-budget trap the surrounding paragraph already warns about.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: TMHSDigital <154358121+TMHSDigital@users.noreply.github.com>

Author: TMHSDigital

showcase/README.md

@@ -65,6 +65,13 @@ entry in `showcase/gallery.json`, and a rendered still.
   the last one over faces that share **no** vertex: the triangles of one
   flat fan cap are coplanar and close-centred by construction, and
   counting those makes the budget unsatisfiable rather than meaningful.
+  Count the coplanar pairs **cross-shell**, not merely over faces that
+  share no vertex: two quads two steps apart on one flat cap share no
+  vertex and are coplanar and close-centred by construction. Counting
+  those reported 96 pairs on `hay-bale`, none of them a hazard, which is
+  the same unsatisfiable-budget trap in a second disguise. Z-fighting is
+  two separate bodies landing on one plane, and that is a cross-shell
+  pair by definition.
   Multi-body props do **not** require Euler characteristic 2 — that is a
   single-shell contract. Two boxes that share a coplanar face z-fight;
   two that share a vertex weld into one shell. Seat a joint with a named
@@ -100,6 +107,26 @@ entry in `showcase/gallery.json`, and a rendered still.
   hypot-length box rotated about its centroid. The centroid form is
   short of one host and punches through the other. `--short-brace` /
   `--float-spout` (or the piece's equivalent) is the falsifier.
+- **Even shaping terms and mirror symmetry (exit 19).** Where a prop has
+  mirrored members — two belts, two brackets, a pair of feet — every
+  closed-form term in the host's shaping function must be **even** in
+  the mirrored axis. `hay-bale` used `sin(x*24)`, which is odd, so its
+  two belt stations sampled different surface heights and the wraps
+  shipped 5.4 mm out of step; `cos` fixed it exactly. Assert it: pair the
+  mirrored shells and compare their axis positions and extents within a
+  named epsilon. A bounding box cannot see this, and neither can a
+  per-member budget that only ever looks at one member.
+- **Wrappers follow the host's profile, and the host is finished first
+  (exit 16/18).** Build, bevel and **ground the host, then** hang the
+  wrapper on the finished surface by raycasting the host's own
+  cross-section at the wrapper's station. Placing a wrapper against
+  half-built geometry and shifting everything afterwards leaves each one
+  a different distance off the floor. A constant-section rectangle on a
+  rounded host stands proud at the middle of each face and is swallowed
+  at the corners — `hay-bale`'s belts stopped 2.9–8.3 mm above the floor
+  with a visible notch, while the AABB `zmin` gate stayed green because
+  the loaf grounded the box. Assert **per wrapper shell** that the loop
+  passes under the host, not just that something touches Z=0.
 - **Seat conformance (exit 18).** A band, hoop, strap or collar wrapped
   around a host asserts a **banded** seat depth — a minimum so it cannot
   float and a maximum so it cannot sink — sampled per angular segment
@@ -110,6 +137,14 @@ entry in `showcase/gallery.json`, and a rendered still.
   vertices as inner ones the moment the host is out of round, which is
   how a hoop that visibly gapped on one side still passed.   The paired
   falsifier makes the wrapper a true circle on an out-of-round host.
+  Measure the depth **station-locally**, along the vertex's own direction
+  from the station axis — not by nearest-surface distance. A wrapper
+  seated in a concave waist has its nearest host face on the bulge
+  shoulder at a neighbouring station, and the wrap then reads as sitting
+  10 mm outside a host it is in fact hugging. Bin the samples by
+  **rounding** to the nearest station, never by flooring: a station on a
+  bin boundary puts its inner and outer rings in different bins, and the
+  inner-only bin reports the wrapper's outer offset as its seat depth.
 - **Level sole on a raked leg (exit 18).** A raked stile does not sit
   in a world-axis cube. Build the sleeve in the member frame so it
   follows the rake; add the tread after the rake as its own