feat(confidence-engine): establish current understanding synthesis seam

app/api/cases/synthesis/route.js

@@ -0,0 +1,68 @@
+/**
+ * Dedicated Current Understanding synthesis API route.
+ *
+ * Route: POST /api/cases/synthesis
+ *
+ * Follows the thin route pattern established by cases/start and cases/update routes:
+ *   parse request → invoke domain seam → return validated result → map failure status
+ *
+ * No synthesis business logic belongs in this file.
+ */
+
+import { getProvider } from "@/lib/llm/provider.js";
+import { synthesizeCurrentUnderstanding } from "@/lib/graph/current-understanding-synthesis.js";
+
+export async function POST(request) {
+  try {
+    const body = await request.json();
+
+    // ── Parse / validate input contract ───────────────────────
+    if (!body || typeof body !== "object") {
+      return Response.json(
+        { success: false, stage: "request_validation", error: "Invalid request body" },
+        { status: 400 }
+      );
+    }
+
+    const { situationGraph, findings } = body;
+
+    if (!situationGraph) {
+      return Response.json(
+        { success: false, stage: "request_validation", error: "Missing situationGraph" },
+        { status: 400 }
+      );
+    }
+
+    // ── Invoke domain seam with provider resolution ───────────
+    const result = await synthesizeCurrentUnderstanding(
+      { situationGraph, findings },
+      { provider: getProvider() }
+    );
+
+    return Response.json({ success: true, currentUnderstanding: result.currentUnderstanding }, { status: 200 });
+  } catch (error) {
+    if (error instanceof SyntaxError) {
+      return Response.json(
+        { success: false, stage: "request_validation", error: "Invalid JSON request body" },
+        { status: 400 }
+      );
+    }
+
+    if (error.statusCode) {
+      return Response.json(
+        {
+          success: false,
+          stage: error.statusCode === 400 ? "request_validation" : "provider",
+          error: error.message ?? "Synthesis failed",
+        },
+        { status: error.statusCode }
+      );
+    }
+
+    // Unexpected error
+    return Response.json(
+      { success: false, stage: "internal", error: "Internal server error" },
+      { status: 500 }
+    );
+  }
+}