Register scripted image definitions in ACAD_IMAGE_DICT and reuse per file

A scripted RasterImage now reuses one ImageDefinition per file and registers
it in ACAD_IMAGE_DICT, which owns it, as for an AutoCAD-attached image. The
reactor is deliberately not written because its owner convention cannot be
verified. An ignored spike records that OCS's own IMAGE command creates bare
images that lose their file path on save.

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

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

@@ -185,3 +185,4 @@ exact queue above and update statuses with evidence after each increment.
 - 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.
+- Improved `RasterImage` linkage. A scripted image now reuses one definition per file and registers it in `ACAD_IMAGE_DICT`, which owns it; this persists through DWG and DXF. The `IMAGEDEF_REACTOR` is deliberately not written: its owner convention is ambiguous between cadcodec's struct docs and its DXF writer and cannot be checked against AutoCAD here, so the kind stays integration-tested. An exploratory spike also showed that OCS's own IMAGE command creates bare images that lose their file path on save, an OCS bug outside this plugin work that is flagged separately.