Draft cadcodec issues for the DXF reader gaps; fix Spline rational flag

Documents six cadcodec reader/writer gaps with source references, canary
tests and suggested fixes (nothing filed). Verifying them showed the Spline
DXF weight loss was an OCS bug: the writer emits weights only for a rational
spline, so the host now sets flags.rational from the weights. Spline passes
every gate.

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

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

@@ -9,7 +9,7 @@ As of 20 September 2026, the working branch is `plugin/host-model-api` at
 [#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 (the legacy `Polyline` is 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.
+kinds: Tolerance, Shape, AttributeEntity, Hatch, MLine, Dimension, MultiLeader, Table, PolygonMesh, PolyfaceMesh, Mesh, Wipeout, Point, Line, Circle, Arc, Ellipse, LwPolyline, Spline, 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 and Polyline3D 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
@@ -174,3 +174,4 @@ exact queue above and update statuses with evidence after each increment.
 - 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.
+- 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.