fix(confidence-engine): project processing indicator to active follow-up block

Repair LOCATION-A defect where processing feedback rendered near the
completed narrative instead of inside the active follow-up block.

Changes:
  - components/reasoning-workspace.jsx: three targeted edits using a single
    spinner component with conditional rendering; hasActiveFollowUp routes
    ownership to the correct container
  - tests/open-questions-vs-assumptions.test.jsx: regression test confirming
    exactly one indicator, DOM child of follow-up-block, ownership separation

Accepted criteria met:
  ✅ Exactly one processing indicator during follow-up processing
  ✅ Indicator is a DOM child of follow-up-block
  ✅ Top-level indicator suppressed when follow-up active
  ✅ Initial answer flow preserved (top-level when no follow-up)
  ✅ Successful follow-up promotion intact
  ✅ All existing context retained
  ✅ No new state/lifecycle changes/error redesign

tests/open-questions-vs-assumptions.test.jsx

@@ -2362,6 +2362,62 @@ describe("v0.49 RENDERED — in-place follow-up context ownership", () => {
       const questionHeadings = screen.queryAllByText(/^Question$/);
       expect(questionHeadings).toHaveLength(0);
     });
+
+    it("processing indicator belongs to active follow-up block, not to completed narrative", async () => {
+      const prevQ = "Which specific step of the onboarding funnel has the highest abandonment rate?";
+      const prevA = "Step 3 — account verification / phone confirmation.";
+      const followUpQ = "What drives the Step 3 abandonment rate?";
+
+      const contrib = makeContrib(prevQ, prevA, 1);
+
+      renderFQB({
+        focused: {
+          question: followUpQ,
+          answer: null,
+          status: "formulated",
+          result: {
+            observations: [contrib.observations[0]],
+            uncertainties: ["Is this causal?"],
+            possibleFollowUpQuestions: [followUpQ, "How does it compare to competitors?"],
+            assumptions: [],
+            relationships: [],
+          },
+          error: null,
+        },
+        focusedContributions: [contrib],
+        processingStep: "active",
+        deconstructMsg: "Working through your response…",
+      });
+
+      // ── Derived completed context (from prior contributions) remains visible during processing ──
+      expect(screen.getByText("Previously answered")).toBeInTheDocument();
+      expect(screen.getByText(prevQ)).toBeInTheDocument();
+      expect(screen.getByText("What this tells us")).toBeInTheDocument();
+
+      // ── Exactly one processing indicator exists (no duplicates, no missing) ──
+      const allProcessingIndicators = screen.queryAllByTestId("processing-indicator");
+      expect(allProcessingIndicators).toHaveLength(1);
+      const processingIndicator = allProcessingIndicators[0];
+
+      // ── Processing indicator must be inside the active follow-up block (Q4) ──
+      const followUpBlock = screen.getByTestId("follow-up-block");
+      expect(followUpBlock.contains(processingIndicator)).toBe(true);
+
+      // ── Processing text visible inside follow-up block ──
+      expect(processingIndicator.textContent).toContain("Working through your response…");
+
+      // ── Follow-up textarea and submit are present inside the same block ──
+      const followUpTextarea = screen.getByTestId("follow-up-textarea");
+      expect(followUpTextarea.disabled).toBe(true);
+      const followUpSubmit = screen.getByRole("button", { name: /submit/i });
+      expect(followUpSubmit.closest("[data-testid='follow-up-block']")).toBeInTheDocument();
+
+      // ── Completed narrative that DOES render must not contain follow-up elements ──
+      const completedNarrative = screen.queryByTestId("completed-narrative");
+      if (completedNarrative) {
+        expect(completedNarrative.querySelector('[data-testid="follow-up-block"]')).not.toBeInTheDocument();
+      }
+    });
   });
 
   // ── v0.51 CASE B: error preserves workspace context ────────────────