fix(confidence-engine): permit empty plausibleInterpretations for honest zero state

- Zod validator: plausibleInterpretations accepts empty string (zero interpretations) while remaining required and string-typed
- Prompt instruction: SECTION B now explicitly requests "" instead of absence prose when no plausible inputs exist
- Report generation/update paths derive hasPlausibleInterpretations from actual synthesized data rather than hard-coding true
- Tests: updated validation contract, added hasPlausibleInterpretations derivation assertions, updated seam tests to use derived values

tests/graph/investigation-overview-synthesis.test.js

@@ -235,9 +235,18 @@ describe("investigation overview — output validation contract", () => {
     expect(result.valid).toBe(false);
   });
 
-  it("empty string plausibleInterpretations rejected", () => {
+  it("empty string plausibleInterpretations accepted (zero interpretations)", () => {
     const result = validateOverviewResponse({ understanding: "something", plausibleInterpretations: "" });
-    expect(result.valid).toBe(false);
+    expect(result.valid).toBe(true);
+    expect(result.data.plausibleInterpretations).toBe("");
+  });
+
+  it("valid empty-string plausibleInterpretations with non-empty understanding is canonical zero", () => {
+    const result = validateOverviewResponse({
+      understanding: "Supported understanding.",
+      plausibleInterpretations: "",
+    });
+    expect(result.valid).toBe(true);
   });
 
   it("malformed JSON rejected", () => {
@@ -245,17 +254,61 @@ describe("investigation overview — output validation contract", () => {
     expect(result.valid).toBe(false);
   });
 
-  it("null input rejected", () => {
-    expect(validateOverviewResponse(null).valid).toBe(false);
-  });
-
-  it("undefined input rejected", () => {
-    expect(validateOverviewResponse(undefined).valid).toBe(false);
+  it("non-empty plausible synthesis remains valid", () => {
+    const result = validateOverviewResponse({
+      understanding: "Supported understanding.",
+      plausibleInterpretations: "Y is still a candidate.",
+    });
+    expect(result.valid).toBe(true);
   });
 
   it("non-object input rejected", () => {
     expect(validateOverviewResponse("just a string").valid).toBe(false);
   });
+
+  // ── hasPlausibleInterpretations derivation from validated data ──
+
+  describe("hasPlausibleInterpretations derivation", () => {
+    it("zero plausible output derives hasPlausibleInterpretations === false", () => {
+      const result = validateOverviewResponse({
+        understanding: "Supported understanding.",
+        plausibleInterpretations: "",
+      });
+      expect(result.valid).toBe(true);
+      const hasPlausibleInterpretations = result.data.plausibleInterpretations.trim().length > 0;
+      expect(hasPlausibleInterpretations).toBe(false);
+    });
+
+    it("non-zero plausible output derives hasPlausibleInterpretations === true", () => {
+      const result = validateOverviewResponse({
+        understanding: "Supported understanding.",
+        plausibleInterpretations: "Some interpretation.",
+      });
+      expect(result.valid).toBe(true);
+      const hasPlausibleInterpretations = result.data.plausibleInterpretations.trim().length > 0;
+      expect(hasPlausibleInterpretations).toBe(true);
+    });
+
+    it("whitespace-only plausibleInterpretations derives false", () => {
+      const result = validateOverviewResponse({
+        understanding: "Supported.",
+        plausibleInterpretations: "   ",
+      });
+      expect(result.valid).toBe(true);
+      const hasPlausibleInterpretations = result.data.plausibleInterpretations.trim().length > 0;
+      expect(hasPlausibleInterpretations).toBe(false);
+    });
+  });
+
+  // ── SECTION B empty-input prompt contract ──
+
+  describe("SECTION B empty-input prompt instruction", () => {
+    it("prompt explicitly permits and requests empty-string output when no plausible inputs", () => {
+      const prompt = buildOverviewSynthesisPrompt(graph, eligibleFindings, []);
+      expect(prompt).toContain('plausibleInterpretations');
+      expect(prompt).toMatch(/empty string/i);
+    });
+  });
 });
 
 // ── 6. No decision/recommendation/readiness fields allowed ───────────────