feat(FN-2255): add plugin runtime discovery contracts

- Extend core/plugin-sdk types with runtime manifest metadata, runtime factory, and runtime registration exports
- Add runtime validation in plugin manifest parsing, including runtimeId slug and semver checks
- Add PluginLoader.getPluginRuntimes() and PluginRunner runtime cache/invalidation plumbing across plugin lifecycle events
- Expand plugin loader/runner test coverage for runtime discovery and cache behavior, and document runtime registration in PLUGIN_AUTHORING.md

packages/plugin-sdk/src/index.ts

@@ -49,6 +49,9 @@ export type {
   PluginRouteDefinition,
   PluginRouteMethod,
   PluginUiSlotDefinition,
+  PluginRuntimeManifestMetadata,
+  PluginRuntimeFactory,
+  PluginRuntimeRegistration,
   PluginContext,
   PluginLogger,
   FusionPlugin,