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/plugin.toml

@@ -8,7 +8,9 @@ license = "GPL-3.0-only"
 repository = "your-account/your-plugin-repo"
 
 [opencad]
-api_version = 5
+# Keep in sync with the Rust manifest's `ApiVersion::CURRENT` — this is the
+# pre-load gate; the runner also checks the compiled-in symbol.
+api_version = 10
 # Replace both placeholders in the packaging step. The v1 template's
 # release workflow contains a working example.
 rustc_version = "__RUSTC_VERSION__"
@@ -21,3 +23,7 @@ command_prefixes = [
     "COUNT_MARKED_SURVEY_POINTS",
 ]
 xdata_apps = ["SURVEYMARK"]
+# NOTE: the plugin's entity-class display labels (see `ENTITY_CLASSES` in
+# src/lib.rs) travel on the registered manifest over IPC, not in this file —
+# they only apply while the plugin is loaded, so plugin.toml (the pre-load
+# discovery file) deliberately does not carry them.