feat(plugins): include rustc version in plugin compatibility gate

OpenCADStudio/plugins#901

- Record the host's rustc --version output in the embedded version_info.json.
- Add rustc_version/rustc_declared/rustc_compatible to plugin manifests and
  release metadata, mirroring the existing acadrust gate.
- Reject API v4+ plugins whose declared rustc differs from the host, with a
  clear one-line error message.
- Propagate the rustc gate through the marketplace, install flow, and UI.
- Update plugin architecture docs to describe the new ABI metadata.

crates/ocs_plugin_api/ARCHITECTURE.md

@@ -50,10 +50,11 @@ The host and each plugin run in separate OS processes. The host re-executes itse
 - A plugin built against major `N` runs on a host whose major is `>= N` because new vtable entries and enum variants are appended at the end.
 - V4 introduces the **acadrust gate**: plugins targeting API v4 or later must resolve the same `acadrust` source as the host (see [`src/version_info.rs`](src/version_info.rs)).
 
-The runtime enforces two gates:
+The runtime enforces three gates:
 
 1. `ocs_plugin_api_version()` exported by the cdylib must be within `[API_VERSION_MIN_SUPPORTED, effective_max_api_version()]`.
 2. For v4+, `acadrust_sources_compatible(host_acadrust_source(), plugin_acadrust_source)` must be true.
+3. For v4+, when the plugin declares `rustc_version`, `rustc_versions_compatible(host_rustc_version(), plugin_rustc_version)` must be true.
 
 ---
 
@@ -215,7 +216,7 @@ Broadcasting is V4-gated inside `PluginManager::broadcast_notification`: V2/V3 p
 The crate embeds two JSON blobs at build time:
 
 - **Type registry** (`OUT_DIR/type_registry.json`) — a language-binding-friendly schema for a curated allow-list of `acadrust` types, generated by `serde-reflection` in `build.rs`. See [`src/type_registry.rs`](src/type_registry.rs).
-- **Version info** (`OUT_DIR/version_info.json`) — host version, `ocs_plugin_api` version, `acadrust` version and source, API versions, build timestamp. See [`src/version_info.rs`](src/version_info.rs).
+- **Version info** (`OUT_DIR/version_info.json`) — host version, `ocs_plugin_api` version, `acadrust` version and source, `rustc` version, API versions, build timestamp. See [`src/version_info.rs`](src/version_info.rs).
 
 Both are accessible without enabling the `host` feature.
 
@@ -242,6 +243,7 @@ This is defense-in-depth, not a sandbox: plugins execute native code.
 | Spawn timeout or crash | Spawn fails; plugin not loaded; error surfaced in UI/logs. |
 | Version mismatch | Host refuses plugin before running plugin code. |
 | Acadrust source mismatch (v4+) | Host refuses plugin to avoid binary incompatibilities. |
+| Rustc mismatch (v4+) | Host refuses plugin to avoid cross-compiler ABI crashes. |
 | Unknown notification discriminant | Deserializes to `HostNotification::Unknown`; plugin can ignore. |
 
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