Add kernel solid operations and complete Solid3D (create and rigid move)

An additive HostApi::solid_operation builds cadkernel primitives and applies
rigid transforms, verifying each result lifts back losslessly and refusing a
payload it cannot lift. Exposed as doc.solids in Python; Solid3D, Body and
Region are mapped read-only with the opaque payload preserved. The lifecycle
test passes through the real runner with analytic volumes and exact extents as
oracles, DWG and DXF reopen with re-edit, and undo restores exactly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

plugins/opencad-python/README.md

@@ -94,7 +94,7 @@ The host-owned catalog classifies all 48 entity variants and tracks every
 traced field, including fields marked `unmapped`. Query it with
 `doc.coverage()` or `doc.coverage("Line")`; each row includes source field,
 type, access, and current validation status. Geometry edits remain limited to
-the 38 mapped kinds.
+the 41 mapped kinds.
 For any entity, `entity.snapshot` returns a detached dictionary of its raw
 CAD fields, including `snapshot["common"]` and kind-specific nested values.
 It can inspect an unmapped kind; editing the dictionary does