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).

changelogs/unreleased.md

@@ -52,6 +52,9 @@
 * **Selection Event Streaming:** Automation clients now receive real-time selection change notifications. *(Contributed by [@YsDirector](https://github.com/YsDirector))*
 * **Host API v8:** Added name-addressed `XRecord` CRUD operations on `HostApi`. *(Contributed by [@schoeller](https://github.com/schoeller))*
 
+#### Plugins
+* **Plugin-Declared Entity Class Labels (Host API v10):** Plugins can now give their XDATA-stamped entities human type names: the plugin stamps a stable class-identity key (e.g. `CVIFC-CLASS:LINE`) into entity XDATA and declares key → label mappings via the new `BuiltinPlugin::entity_classes()` manifest accessor; while the plugin is loaded, the Properties panel title and the records/automation `display_type` field show `<Label> (<Native>)` (e.g. "Linear Element (Line)"). Labels are display-only, resolved live from the loaded plugin set (not installed / disabled / dead process → bare native type), and never round-trip the drawing — identity keys do, so labels stay plugin-localizable without rewriting files. Raw schema fields (`type`, `dxf_name`, `record_type`) and the `LIST` command output are unchanged. *(Contributed by [@schoeller](https://github.com/schoeller))*
+
 ---
 
 ### 🐛 Bug Fixes