Exercise document model lifecycle through plugin IPC

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

@@ -104,3 +104,4 @@ Start with the **Phase 8 integration harness and coverage ledger**. Re-evaluate
 - The same Line test now also sends a duplicate-handle batch through Python and confirms the host rejects it without changing geometry or creating an undo entry.
 - The opt-in `staged_python_point_pick_and_cancel_over_real_ipc` test passes with the same runner. It requests point and entity picks through Python, drives the app's active-command point/object-pick callbacks, polls the returned tokens through a later Python run, and verifies Enter cancellation. The callbacks are invoked by the test harness, not by a physical GUI pointer event; live GUI verification remains open.
 - The opt-in `staged_python_tabs_isolate_tokens_and_notifications` test passes with two real OCS document tabs and the staged runner. A token created in tab 0 cannot be observed or consumed in tab 1 before or after completion. Host-to-plugin DrawingChanged notifications for both tab ids are delivered over IPC and drained only by their owning tab. A 257-event burst reports overflow, and `DocumentTabClosed` discards queued events for that tab.
+- Added `doc.create_entity(kind, **properties)` and `doc.delete_entity(entity_or_handle)` to the high-level Python document model. Unit tests cover forwarding and managed identity fields. The real staged-plugin Line lifecycle now uses these methods rather than low-level `ocs.add`/`remove_entity`.

docs/plugin-host-model-coverage-ledger.md

@@ -7,7 +7,7 @@ Completion gate codes: **C** = create through Python document model; **R** = rea
 | Kind | Baseline | Gate evidence / blocker |
 |---|---|---|
 | Point | Mapped | Host transaction test now checks invalid batch rollback, grouped edit undo, and redo; creation/deletion history, real plugin execution, GUI, and DWG/DXF gates remain pending. |
-| Line | Mapped | Actual staged-plugin IPC Python create/edit/delete, three undo entries and undo/redo, and edited DWG/DXF reopen pass in `staged_python_plugin_line_lifecycle_over_real_ipc`. GUI pick/visibility, invalid-input/reference preservation, and document-model create/delete API still pending; not complete. |
+| Line | Integration-tested | High-level document-model create/delete and transaction edit pass through the actual staged-plugin IPC runner; three undo entries, undo/redo, duplicate-handle atomic rejection, and edited DWG/DXF reopen pass in `staged_python_plugin_line_lifecycle_over_real_ipc`. GUI pick/visible-geometry observation and broader invalid-value/unmapped-field preservation checks remain pending; not complete. |
 | Circle | Mapped | Full gate pending. |
 | Arc | Mapped | Full gate pending. |
 | Ellipse | Mapped | Existing focused DWG conversion test; full gate pending. |