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

plugins/fusion-plugin-reports/docs/rendering-contract.md

@@ -0,0 +1,81 @@
+## ReportRecord input shape
+
+```ts
+interface ReportRecord {
+  id: string;
+  title: string;
+  cadence: "daily" | "weekly" | "monthly" | "quarterly" | "manual";
+  period: { start: string; end: string };
+  generatedAt: string;
+  status: string;
+  settings: {
+    sections?: string[];
+    sectionOrder?: string[];
+    branding?: {
+      accentColor?: string;
+      logoDataUri?: string;
+    };
+  };
+  sections: {
+    summary?: string;
+    system?: {
+      wins?: string[];
+      highlights?: string[];
+      lowlights?: string[];
+      proposals?: string[];
+      deepDives?: string[];
+    };
+    perAgent?: Array<{
+      agentId: string;
+      agentName?: string;
+      wins?: string[];
+      highlights?: string[];
+      lowlights?: string[];
+      proposals?: string[];
+      deepDives?: string[];
+    }>;
+    dataCoverage?: string[];
+  };
+  reviewPanel?: {
+    overallVerdict: string;
+    consensusSummary: string;
+    individual: Array<{ memberName: string; verdict: string }>;
+  };
+}
+```
+
+## Section identifiers
+
+- `data-section="summary"`
+- `data-section="system-wins"`
+- `data-section="system-highlights"`
+- `data-section="system-lowlights"`
+- `data-section="system-proposals"`
+- `data-section="system-deep-dives"`
+- `data-section="agent-card"`
+- `data-section="data-coverage"`
+- `data-section="review-panel"`
+
+## Theme tokens
+
+- `--space-xs`, `--space-sm`, `--space-md`, `--space-lg`, `--space-xl`
+- `--radius-sm`, `--radius-md`, `--radius-lg`
+- `--bg`, `--surface`, `--card`, `--text`, `--text-muted`, `--border`
+- `--triage`, `--todo`, `--in-progress`, `--in-review`, `--done`
+- `--color-success`, `--color-error`, `--color-warning`, `--color-info`
+- `--report-accent`
+
+## HTTP endpoints
+
+- `GET /api/plugins/reports/reports/:id/export.html`
+  - `200 text/html; charset=utf-8` + `Content-Disposition: attachment; filename="<slug>.html"`
+  - `404` when report ID does not exist
+  - `409` when report is not yet generated
+- `GET /api/plugins/reports/reports/:id/preview.html`
+  - `200 text/html; charset=utf-8` body-only fragment (`<article>...</article>`)
+  - `404` when report ID does not exist
+  - `409` when report is not yet generated
+
+## Stability
+
+This rendering contract is shared with downstream dashboard/share integrations (FN-3786 / FN-3787). Any breaking change to markers, token names, or endpoint contract requires coordinated updates across those tasks.