Design: slot Radius caption, keyboard offer, plane combo removal, mass props, docs

Mirror of snaporca ac85277bac..0e7cb3ec78 (six changes, applied as a patch to
DesignPanel.cpp rather than copied, so this fork's 30 permitted divergent lines survive
— parity re-checked afterwards: the five shared files are byte-identical, DesignCanvas.cpp
and DesignPanel.cpp differ by exactly 16 and 30 lines).

- The straight slot's inline field says Radius, which is what it sets. It stores the
  half-width and passed the typed number through unchanged, so 30 produced a 60 mm slot.
- The offer opens from the keyboard (Menu, Shift+F10), anchored on the viewport rather
  than wherever the pointer happens to be. The card hint names the new route.
- The sketch card's Plane combo is gone; the plane comes from the viewport. Also stops
  build_candidate collapsing a face plane to a base plane while editing.
- Mass properties and the dead Edit row are wired into the offer; DesignOffer.hpp is
  regenerated from tool_atlas.json, verified by re-running the generator and diffing.
- docs/rig_build_traps.md + scripts/rig-build.sh, which derives its fork identity from
  project() so it cannot be pointed at the other fork's image or volume.
- docs/design_tab.md refreshed (44 commits stale) + a PR description, with this fork's
  own merge-base and diff shape rather than snaporca's.

Built green in the deps container with the new script and verified on the rig: Menu and
Shift+F10 both open the offer at the viewport centre with the pointer parked off-canvas,
and the sketch card shows no Plane row.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LyRwbuq6fjn3VV9U9UvhBM

docs/ux/tool_atlas.json

@@ -829,8 +829,8 @@
         "bodies": 1
       },
       "refusal": null,
-      "gui": false,
-      "action": null
+      "gui": true,
+      "action": "btn:mass"
     },
     {
       "id": "interference",
@@ -871,7 +871,7 @@
       "needs": {},
       "refusal": null,
       "gui": true,
-      "action": null
+      "action": "btn:edit"
     },
     {
       "id": "delete_face",