Complete Viewport: layout-tab canvas oracle, unique ids, derived scale

The shared driver now activates the layout that owns a viewport and requires
a drawn frame. The host assigns a unique id of at least 2 and derives
custom_scale as height / view_height on every scripted create or edit, as
MVIEW does, so custom_scale is read-only. A control showed the default id 0
is not invisible, so the id matters for uniqueness.

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

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

@@ -9,11 +9,11 @@ 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 **43 of 43** canvas kinds: 41
 are creatable (the legacy `Polyline` 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, Surface and Ole2Frame 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.
+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, Surface, Ole2Frame and Viewport 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, 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
+`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 is held short of `Complete` by unverifiable image-dictionary linkage (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.
 
@@ -184,3 +184,4 @@ exact queue above and update statuses with evidence after each increment.
 - 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.
 - Completed `Ole2Frame` by adding creation. `SolidOperation::EmbedPicture` and `doc.embed_picture` build the frame from a picture file with OCS's own embedding code, so a script never touches the opaque storage; PNG, JPEG and BMP are stored byte-for-byte and other formats are re-encoded as PNG. The lifecycle test now creates through Python and the new creation-rules test covers formats, aspect, origin, layer and seven refusals. 30 kinds are complete.
+- Completed `Viewport` by closing its missing canvas oracle and its derived state. The shared driver now switches the scene to the layout that owns a viewport and requires a drawn frame. The host assigns a unique id (at least 2) and derives `custom_scale = height / view_height` on every scripted create or edit, as MVIEW does, so `custom_scale` became read-only. A control showed my earlier suspicion was wrong: the default id 0 still draws a frame in a layout with no sheet viewport, so the id matters for uniqueness, not visibility. 31 kinds are complete.