Investigate the cadkernel path for ACIS kinds; add spikes and findings

OCS already creates, transforms and combines bodies through cadkernel, and
solid_to_sat / kernel_acis_body verify each result lifts back losslessly.
Two ignored spike tests measure volumes and DWG/DXF round trips. The
blocker is an operation channel, not geometry; the note recommends an
additive solid_operation host method and phases the work.

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

docs/plugin-host-model-completion-plan.md

@@ -176,3 +176,4 @@ exact queue above and update statuses with evidence after each increment.
 - Decided the two open audit questions. **Insert is creatable** by naming an existing block (existence, ordinary-block and no-cycle checks; attribute records stay with the AttributeEntity flow), which completes the kind. **Legacy Polyline is update-only** with a hint pointing scripts at Polyline2D and Polyline3D, because both formats convert it on save. The generator gained an `update_only_message` for such kinds, integer coordinates are accepted in Insert scales, and the driver gained `MAKEPOLYLINE` and `MAKEBLOCK` fixtures.
 - Wrote up the cadcodec DXF reader gaps as draft issues in [cadcodec-reader-gaps.md](cadcodec-reader-gaps.md) (nothing filed): ATTDEF, boolean groups read through `as_i16()`, UNDERLAY rotation units, LEADER, DRAWINGVIEW/SECTIONLINE dispatch and TABLE merged ranges. Verifying them corrected an earlier attribution: the Spline "DXF drops weights" finding was an OCS bug (the writer emits weights only for a rational spline), now fixed by setting `flags.rational` from the weights, so Spline is complete.
 - Mapped `Ole2Frame` as update-only (queue item 21, previously blocked). OCS already builds real embedded frames (`build_embedded_ole`), which supplied the fixture the blocker was waiting for. Frame corners and the aspect lock are scriptable; the opaque storage and envelope are excluded and refused on edit. The audit proves the embedded picture survives DWG and DXF unchanged. Creation from Python remains a follow-up. The remaining blocked kinds are SectionSymbol, Region, Body, Solid3D and Surface.
+- Investigated the cadkernel path for the ACIS kinds ([cadkernel-body-path.md](cadkernel-body-path.md)). The earlier "blocked" reasoning was overstated: OCS already builds primitives, transforms bodies and runs booleans through cadkernel, and `solid_to_sat` plus `kernel_acis_body` verify each result lifts back with no loss. Two `#[ignore]` spike tests measured exact volumes and DWG and DXF round trips for a box, moved, rotated and mirrored copies, three booleans, four other primitives and a `Body` entity. The real gap is an operation channel from a script to that pipeline; the note recommends an additive `solid_operation` host method and a phased plan (Solid3D create and transform, then booleans and Body, then Region, then Surface). Booleans took about 23 s each in a debug build and need release timing first. Region, Body, Solid3D and Surface move from blocked to unblocked-by-design; SectionSymbol stays blocked.