docs(showcase): vessel ends are boards too, lapped where they hold water

Extends "a head or bottom is boards, not a slab" to any end that closes a
vessel: lap watertight boards a millimetre rather than leaving a through
seam, and read rim tests on the union of an end's boards.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Signed-off-by: TMHSDigital <154358121+TMHSDigital@users.noreply.github.com>

Author: TMHSDigital

showcase/README.md

@@ -458,6 +458,14 @@ entry in `showcase/gallery.json`, and a rendered still.
   not by a radius: a quarter of the head's width is smaller than any
   "round" test built for the whole disk. `--one-piece-head` /
   `--one-piece-bottom` are the falsifiers.
+
+  The same holds for any end that closes a vessel. `water-trough`'s ends
+  were one D-shaped slab each; they are three vertical boards. Where the
+  boards must also hold water, lap them a millimetre into each other
+  rather than leaving a seam: a through gap leaks, and lets a containment
+  ray through, while the bevel on the boards' face edges still shows the
+  joint. Read any rim test on the union of an end's boards, or a board's
+  inner edge at a joint is taken for the rim (`--slab-ends`).
 - **Rope is laid, not piped.** A smooth tube with a rope material is a
   plastic hose. Loft a three-lobed section and turn it one vertex step per
   ring, so each lobe winds along the path as a strand. That costs the same