Map SectionSymbol, the last unmapped kind

points is canonical; the host derives the point counts and refreshes the
end/tick/label projections with sync_display_fields when it changes, and
validates that style and view-representation references are existing class
objects. The lifecycle test passes every gate in DWG; DXF reopens the symbol
as an unknown entity because cadcodec dispatches SECTIONLINE only inside
blocks (pinned by a canary and already in the cadcodec write-up).

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

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

@@ -7,13 +7,13 @@ Build a general Python document model over the OCS host API for all **43 canvas
 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 **42 of 43** canvas kinds: 39
+v7. The Python feature has explicit writes for **43 of 43** canvas kinds: 40
 are creatable (the legacy `Polyline`, `Ole2Frame` and `Body` are update-only). This is **not** 41 completed
 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, Face3D, Solid3D, Region and Surface 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
-`acadrust` revision `7ea4247`. Leader is mapped and integration-tested but blocked at W by three acadrust persistence gaps (see the ledger). Leader, Helix, Underlay, ViewBorder and Light are mapped and integration-tested but blocked at W by acadrust persistence gaps, and RasterImage and Viewport are held short of `Complete` by unverifiable or missing oracles (see the ledger). RasterImage is mapped and integration-tested, held short of `Complete` by unverifiable image-dictionary linkage. The remaining canvas kind, SectionSymbol, has only `layer` and is recorded as blocked
+`acadrust` revision `7ea4247`. Leader is mapped and integration-tested but blocked at W by three acadrust persistence gaps (see the ledger). Leader, Helix, Underlay, ViewBorder, Light and SectionSymbol are mapped and integration-tested but blocked at W by acadrust persistence gaps, and RasterImage and Viewport are held short of `Complete` by unverifiable or missing oracles (see the ledger). RasterImage is mapped and integration-tested, held short of `Complete` by unverifiable image-dictionary linkage. Every canvas kind now has explicit writes; SectionSymbol is mapped and integration-tested but blocked at DXF persistence by cadcodec
 writes through the document model. All 43 have raw typed snapshots. There are
 three internal records and two opaque fallbacks outside the 43 canvas kinds.
 
@@ -182,3 +182,4 @@ exact queue above and update statuses with evidence after each increment.
 - Completed `Surface` for planes, extruded open profiles, isolines and rigid moves. `SurfaceFromProfile` and `Extrude` join the kernel operations (`doc.solids.surface` and `doc.solids.extrude`); `Extrude` also gives Solid3D extrusions from closed profiles, checked against analytic volumes. The test found and fixed a real bug: Surface's `kind` field silently overwrote the entity-type `kind` key in Python dicts, so it is exposed as `surface_kind`, and a new uniqueness test guards the whole catalog. Transform now accepts surfaces (a plane stays a plane; other kinds turn generic). Left: booleans (release timing first), regions with holes, and lofted, revolved, swept and NURBS surfaces. SectionSymbol is the only unmapped kind.
 - Timed booleans in a release build (details in [cadkernel-body-path.md](cadkernel-body-path.md)). A curved box-and-cylinder boolean takes about 1 to 2.2 s, overlapping spheres 0.2 s and planar operands 1 to 2 ms, against 23 s in a debug build. The kernel refuses many curved cases quickly with a reason (`Coincident`, `CutRefused`, `NoClosedForm`), including a second cut in an already drilled box. Recommendation: add a synchronous `SolidOperation::Boolean` that surfaces the refusal and leaves the operands untouched, test it with planar operands plus one ignored curved case, and note the 30 s per-script budget. No code changed in this step.
 - Implemented `SolidOperation::Boolean` and `doc.solids.union`, `subtract` and `intersect`, as recommended by the timing study. It runs synchronously, surfaces the kernel's refusal (`Coincident`, `CutRefused`, `NoClosedForm`) with a plain-language reason and the words "nothing was changed", consumes the operands unless `keep_operands`, and records one undo step. The lifecycle test uses planar boxes (millisecond cost, exact volumes, debug-safe) and an `#[ignore]` test documents a curved refusal. With this the kernel operations in the cadkernel note are all built. SectionSymbol is the only unmapped kind.
+- Mapped `SectionSymbol`, the last unmapped kind, so all 43 canvas kinds now have explicit writes. The blocker recorded earlier was overstated: cadcodec documents `points` as canonical, says the counts equal the point count in every verified export, and provides `sync_display_fields()`, which OCS's own edits already call; OCS also renders a symbol with no style or view link. The host derives the counts and projections when `points` changes and validates that references are existing `SectionViewStyle` and `ViewRep` class objects. Every gate passes except DXF, where cadcodec reopens a top-level `SECTIONLINE` as an unknown entity (issue 5 in the cadcodec write-up, pinned by a canary). Still unverifiable here: the meaning of `raw_flags_90` and AutoCAD's acceptance of a symbol with no parent view.