feat(FN-3785): add standalone HTML rendering and export routes for reports

Adds a complete HTML rendering and export pipeline to the fusion-plugin-reports plugin, including a standalone HTML renderer with template and stylesheet support, new export routes (`/api/reports/:id/export/html`) that persist rendered HTML to the store, and plugin-type support for non-JSON route re

Fusion-Task-Id: FN-3785

docs/PLUGIN_AUTHORING.md

@@ -455,6 +455,8 @@ const plugin: FusionPlugin = {
 };
 ```
 
+Route handlers may return either plain JSON values or a `PluginRouteResponse` envelope. `PluginRouteResponse` now supports optional `headers` and `contentType` fields so plugins can serve non-JSON payloads like downloadable HTML. Example: return `{ status: 200, body: html, contentType: "text/html; charset=utf-8", headers: { "Content-Disposition": "attachment; filename=\"report.html\"" } }` to send an attachment response directly from a plugin route.
+
 ### Route Mounting
 
 Routes are mounted at `/api/plugins/{pluginId}/{path}`.