Meshing test corpus built with SolidCraft; assembly placement in open_design (#11)

* geometry: build designs from SolidCraft command scripts (feature scripts)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* geometry: place component bodies through their occurrences when opening a design

An assembly's instances were missing and moved components sat at their own origin.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* Meshing test corpus: 16 SolidCraft recipes, analytic shapes, loader and runner

modelcraft-testkit holds the corpus (tests/corpus/<name>/recipe.json run through SolidCraft's
engine, then opened as a design) and the runner binary behind cargo xtask corpus.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* docs: corpus results, test corpus architecture and the scripts feature; licences

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* corpus: measure each body once, check stage widths, simpler loader and runner

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

---------

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

tests/corpus/rounded-box/README.md

@@ -0,0 +1 @@
+A 40 x 30 x 20 box with every edge rounded R3: cylinder and sphere blends meeting at the corners.

tests/corpus/rounded-box/recipe.json

@@ -0,0 +1,5 @@
+{"corpus": {"categories": ["fillet"], "volume_mm3": 23340.849375},
+ "commands": [
+  {"command": "solid.box", "params": {"corner": [0, 0, 0], "length": 40, "width": 30, "height": 20, "body_name": "Box"}},
+  {"command": "solid.fillet", "params": {"edges": [[0, 0, 10.0], [0, 30, 10.0], [40, 0, 10.0], [40, 30, 10.0], [0, 15.0, 0], [0, 15.0, 20], [40, 15.0, 0], [40, 15.0, 20], [20.0, 0, 0], [20.0, 0, 20], [20.0, 30, 0], [20.0, 30, 20]], "radius": 3}}
+ ]}