docs(plugins): entity-class label convention; demo in template v2

- plugin-architecture.md: new "Entity class display labels (API v10)"
  section documenting the split (stable identity keys in XDATA, display
  labels in the plugin declaration), the live-loaded-plugin gating (missing,
  disabled, cap-excluded, failed, removed, or dead → bare native type), the
  "<Label> (<Native>)" display rule, that raw schema fields (`type`,
  `dxf_name`, `record_type`) and LIST are unchanged, and that the label is
  display-only. `plugin.toml` deliberately does not carry the table: it is
  the pre-load discovery file and labels must activate on live manifests.
- Template v2 demonstrates the whole convention: declares
  `SURVEY-CLASS:POINT -> "Survey Point"` and stamps the identity key in
  MARK_FIRST_SURVEY_POINT, so marked points show as "Survey Point (Point)"
  while the plugin is loaded. plugin.toml's api_version is bumped to 10 (keep
  in sync with ApiVersion::CURRENT) with a NOTE on where the class table
  lives. DESIGN.md documents the flow.
- Template v2's stale `acadrust` pin (8a28c21, predating the host's codec
  bump to 278c49a) is realigned with the host's pin: without it the template
  no longer compiles on main — its `acadrust::EntityType` and the host
  re-exports are distinct types when the revisions disagree (verified the
  same four type errors occur before and after this feature's template edit).
- changelogs/unreleased.md entry (Plugins section).

docs/plugin-template-v2/DESIGN.md

@@ -20,7 +20,7 @@ rather than a runtime dependency of the host.
 |---|---|
 | `COUNT_SURVEY_POINTS` | Count point entities on the `SURVEY` layer via `document_reader()`. |
 | `ADD_SURVEY_POINT` | Add a new point entity on the `SURVEY` layer via `host.add_entity()`. |
-| `MARK_FIRST_SURVEY_POINT` | Read the first `SURVEY` point and mark it with an XDATA record. |
+| `MARK_FIRST_SURVEY_POINT` | Read the first `SURVEY` point and mark it with an XDATA record (including the `SURVEY-CLASS:POINT` identity key). |
 | `COUNT_MARKED_SURVEY_POINTS` | Count `SURVEY` points that carry the `SURVEYMARK` XDATA record. |
 
 ---
@@ -48,6 +48,24 @@ Each [`ReaderEntity`](ocs_plugin_api::host::ReaderEntity) exposes the entity
 
 This is the primary read→write round-trip pattern the template demonstrates.
 
+### Entity class display labels
+
+The template also demonstrates the entity-class label convention (API v10):
+
+- the plugin declares `ENTITY_CLASSES` and exposes it through
+  `BuiltinPlugin::entity_classes()`: app `SURVEYMARK`, prefix
+  `SURVEY-CLASS:`, and key → label mappings;
+- `MARK_FIRST_SURVEY_POINT` stamps a `SURVEY-CLASS:POINT` string value into
+  the point's XDATA alongside the existing marker value.
+
+While the plugin is loaded, the host shows marked points as
+"Survey Point (Point)" in the Properties panel title and in the
+`display_type` field of the records/automation APIs; without the plugin they
+are plain "Point" again. Only the stable key round-trips the drawing — the
+label text lives in the plugin and can be reworded or localized without
+touching saved files. The label is display-only: it never governs snapping,
+selection, or any other behavior.
+
 ### Ribbon integration
 
 The plugin registers one ribbon module (`SurveyTools`) with one group