Decide Insert and Polyline: Insert creatable by block name, Polyline update-only

Insert can be created by naming an existing ordinary block; the host refuses
unknown blocks, model/paper space and containment cycles, and keeps legacy
inserts of missing blocks editable. The legacy Polyline is update-only with a
hint toward Polyline2D and Polyline3D because both writers convert it on
save. The generator gains update_only_message.

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

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

@@ -8,8 +8,8 @@ As of 20 September 2026, the working branch is `plugin/host-model-api` at
 `a8dd3640` plus the Leader increment, published as draft PR
 [#1391](https://github.com/HakanSeven12/OpenCADStudio/pull/1391). Host API is
 v7. The Python feature has explicit writes for **37 of 43** canvas kinds: 36
-are creatable and `Insert` remains update-only. This is **not** 37 completed
-kinds: Tolerance, Shape, AttributeEntity, Hatch, MLine, Dimension, MultiLeader, Table, PolygonMesh, PolyfaceMesh, Mesh, Wipeout, Point, Line, Circle, Arc, Ellipse, LwPolyline, Text, MText, Ray, XLine, Solid and Face3D pass every completion gate; the audited Polyline, Polyline2D, Polyline3D, Spline and Insert pass every gate that their format or update-only status allows.
+are creatable (the legacy `Polyline` is update-only) and `Insert` remains update-only. This is **not** 37 completed
+kinds: Tolerance, Shape, AttributeEntity, Hatch, MLine, Dimension, MultiLeader, Table, PolygonMesh, PolyfaceMesh, Mesh, Wipeout, Point, Line, Circle, Arc, Ellipse, LwPolyline, Text, MText, Ray, XLine, Solid and Face3D pass every completion gate; Insert now passes every gate too; the audited Polyline (update-only by decision), Polyline2D, Polyline3D and Spline pass every gate that their format allows.
 AttributeDefinition passes its host and real IPC lifecycle but remains short of
 `Complete` because the previously pinned CAD codec dropped optional ATTDEF
 fields during DXF reads; revalidate that blocker against the current
@@ -173,3 +173,4 @@ exact queue above and update statuses with evidence after each increment.
 - Recorded `Region`, `Body`, `Solid3D`, `Surface` (queue items 17-20) and `Ole2Frame` (item 21) as **blocked**. The ACIS kinds need a host-side kernel-backed create/transform API that regenerates the payload, wires and silhouettes; the OLE frame needs a valid embedded-storage fixture. No opaque payload is rewritten. With this the ordered queue is complete: 12 kinds are fully complete, Leader, Helix, Underlay, ViewBorder and Light are mapped but blocked at DWG/DXF persistence by acadrust gaps, RasterImage and Viewport lack an oracle or linkage, and six kinds are blocked. Next work is the engine fixes behind the canary assertions, the mapped-kind audit of the 17 pre-existing kinds, and merging `origin/main`.
 - Audited the 17 pre-existing mapped kinds through the shared real-runner driver (`audit_python_*`). The audit found and fixed genuine validation gaps: Ellipse had none, Text and MText accepted a zero height, Arc accepted non-finite angles, Point and Line accepted infinite thickness, and most basic kinds had no creation-time validation. A shared `validate_basic_entity` now covers Point, Line, Circle, Arc, Ellipse, Text, MText, LwPolyline, Polyline, Polyline2D, Polyline3D and Spline, refusing only problems an edit introduces so already-invalid legacy values stay editable. Twelve of the 17 kinds now pass every gate (Point, Line, Circle, Arc, Ellipse, LwPolyline, Text, MText, Ray, XLine, Solid, Face3D). Four expose persistence findings pinned by canaries (Polyline reopens as Polyline3D; Polyline2D reopens as LwPolyline in DXF; Polyline3D loses elevation in DWG; Spline loses weights in DXF), and Insert is update-only with all applicable gates passing. The driver gained per-format DWG expectations and a host-made block fixture for Insert.
 - Audited `AttributeDefinition` against acadrust `5b682ed` with a full-field comparison through both formats. Everything passes in DWG. DXF keeps only tag, prompt, value, insertion point, height and rotation because the reader handles eight group codes; alignment, width factor, oblique angle, attribute flags, field length, generation flags, lock and text style reopen as defaults. The kind stays integration-tested and the gap is pinned by canary expectations; the fix belongs in cadcodec's `read_attdef`. The shared driver's block fixture now also serves `MAKEBLOCK` scripts.
+- 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.