diff --git a/lib/fixtures/rto26b-branch-scoped.mjs b/lib/fixtures/rto26b-branch-scoped.mjs
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+/**
+ * RTO.26B — Branch-scoped experimental fixture
+ *
+ * Replicates the RTO.26A branch record inside the UI layer.
+ * Membership is determined by explicit `branchId` provenance only.
+ *
+ * No semantic filtering, keyword matching, or whole-state reconstruction.
+ */
+
+// ── Deterministic ID generator (mirrors RTO.26A harness) ───────────────
+
+function genId(prefix, label) {
+  let h = 0;
+  for (let i = 0; i < label.length; i++) {
+    h = (Math.imul(31, h) + label.charCodeAt(i)) | 0;
+  }
+  return `${prefix}-${(Math.abs(h) % 1e6).toString(16).padStart(6, "0")}`;
+}
+
+// ── Record model (explicit provenance via `branchId`) ─────────────────
+
+function createQuestion(branchId, text) {
+  return {
+    id: genId("question", text.slice(0, 20)),
+    branchId,
+    text,
+    contributions: [], // stores contribution ids
+  };
+}
+
+function createContribution(branchId, questionId, text) {
+  return {
+    id: genId("contribution", text.slice(0, 20)),
+    branchId,
+    questionId,
+    text,
+  };
+}
+
+function createLateResult(branchId, contributionId, text) {
+  return {
+    id: genId("late-result", text.slice(0, 20)),
+    branchId,
+    contributionId,
+    text,
+  };
+}
+
+// ── Fixed fixture data (mirrors RTO.26A stages 1–5) ───────────────────
+
+function buildBranchScopedFixture() {
+  const records = [];
+
+  // Branch A: Competitor development
+  const qA1 = createQuestion(
+    "branch-competitor-development",
+    "What evidence suggests competitors are actively developing similar products?"
+  );
+  records.push(qA1);
+
+  const cA1 = createContribution(
+    "branch-competitor-development",
+    qA1.id,
+    "The competitor has hired machine-learning engineers and presented at an industry conference about the same customer problem."
+  );
+  records.push(cA1);
+  qA1.contributions.push(cA1.id);
+
+  const qA2 = createQuestion(
+    "branch-competitor-development",
+    "What would clarify whether this activity is product development rather than general market positioning?"
+  );
+  records.push(qA2);
+
+  // Branch B: Customer demand
+  const qB1 = createQuestion(
+    "branch-customer-demand",
+    "What evidence do we have that customers will actually buy the product?"
+  );
+  records.push(qB1);
+
+  const cB1 = createContribution(
+    "branch-customer-demand",
+    qB1.id,
+    "One enterprise customer has expressed strong interest but has not yet signed a contract."
+  );
+  records.push(cB1);
+  qB1.contributions.push(cB1.id);
+
+  const qB2 = createQuestion(
+    "branch-customer-demand",
+    "How much revenue from other customers is sufficiently committed or probable?"
+  );
+  records.push(qB2);
+
+  // Late result scoped to Branch A / Contribution A1 (stage 5)
+  const lateResultA1 = createLateResult(
+    "branch-competitor-development",
+    cA1.id,
+    "This later interpretation relates to Contribution A1."
+  );
+  records.push(lateResultA1);
+
+  return {
+    // Records keyed by branch provenance
+    allRecords: records,
+    questions: {
+      a1: qA1, a2: qA2, b1: qB1, b2: qB2,
+    },
+    contributions: { a1: cA1, b1: cB1 },
+    lateResults: { a1: lateResultA1 },
+  };
+}
+
+// ── Branch-scoped retrieval (explicit provenance only) ─────────────────
+
+/**
+ * getBranchReasoning(records, branchId)
+ * Returns ONLY records whose `branchId` matches. No semantic filtering.
+ */
+function getBranchReasoning(allRecords, branchId) {
+  return allRecords.filter(r => r.branchId === branchId);
+}
+
+// ── Branch-to-label mapping (UI layer only — not provenance logic) ─────
+
+const BRANCH_META = {
+  "branch-competitor-development": {
+    id: "branch-a",
+    label: "Competitor development",
+    origin: "Whether competitors are developing similar products and when they might release them",
+  },
+  "branch-customer-demand": {
+    id: "branch-b",
+    label: "Customer demand",
+    origin: "How many customers will buy the product, and what revenue that represents",
+  },
+};
+
+// ── Exported interface for UI consumption ────────────────────────────────
+
+export function useBranchScopedFixture() {
+  const fixture = buildBranchScopedFixture();
+  const allRecords = fixture.allRecords;
+
+  return {
+    /** Return branch-local question records as node-like objects. */
+    getBranchQuestions(branchId) {
+      const recs = getBranchReasoning(allRecords, branchId);
+      return recs
+        .filter(r => !r.questionId && !r.id.startsWith("late"))
+        .map(q => ({
+          id: q.id,
+          label: q.text,
+          kind: "unknown",
+          status: "unresolved",
+          _isFixtureQuestion: true,
+          contributions: q.contributions || [],
+        }));
+    },
+
+    /** Return branch-local contribution records scoped to active branch. */
+    getBranchContributions(branchId) {
+      const recs = getBranchReasoning(allRecords, branchId);
+      return recs.filter(r => r.questionId && !r.id.startsWith("late"));
+    },
+
+    /** Check if a late result exists for the given contribution. */
+    hasLateResult(contributionId) {
+      return Object.values(fixture.lateResults).some(lr => lr.contributionId === contributionId);
+    },
+
+    /** Return late results scoped to a specific branch (for display). */
+    getBranchLateResults(branchId) {
+      const recs = getBranchReasoning(allRecords, branchId);
+      return recs.filter(r => r.id.startsWith("late"));
+    },
+
+    /** Branch metadata for UI labels. */
+    getBranchMeta(id) {
+      // Reverse lookup: UI id → provenance branchId → meta
+      const entry = Object.entries(BRANCH_META).find(
+        ([, m]) => m.id === id
+      );
+      return entry ? { ...entry[1] } : null;
+    },
+
+    /** Return the fixed two-branch list for the switcher. */
+    getBranches() {
+      return Object.values(BRANCH_META).map(m => ({ ...m }));
+    },
+
+    /** Return all records (for verification / debugging). */
+    getAllRecords() {
+      return allRecords;
+    },
+
+    /** Verify that a record's provenance matches its branch. */
+    verifyProvenance(record, expectedBranchId) {
+      return record?.branchId === expectedBranchId;
+    },
+
+    // ── RTO.26A cross-cutting invariants (deterministic checks) ──────
+
+    _crossCuttingInvariants() {
+      const branchAIds = new Set(getBranchReasoning(allRecords, "branch-competitor-development").map(r => r.id));
+      const branchBIds = new Set(getBranchReasoning(allRecords, "branch-customer-demand").map(r => r.id));
+
+      return {
+        SEMANTIC_FILTERING_REQUIRED: false,
+        KEYWORD_MATCHING_REQUIRED: false,
+        WHOLE_STATE_RECONSTRUCTION_REQUIRED: false,
+        GLOBAL_SELECTOR_REQUIRED: false,
+        CROSS_BRANCH_CONTAMINATION: [...branchAIds].some(id => branchBIds.has(id)),
+        NEXT_QUESTION_SELECTED: false,
+      };
+    },
+
+    // Expose fixture for inspection (RTO.26A stages 1–5)
+    _fixtureRecords: allRecords,
+    _qA1: fixture.questions.a1,
+    _qA2: fixture.questions.a2,
+    _qB1: fixture.questions.b1,
+    _qB2: fixture.questions.b2,
+    _cA1: fixture.contributions.a1,
+    _cB1: fixture.contributions.b1,
+    _lateResultA1: fixture.lateResults.a1,
+  };
+}
+
+export { buildBranchScopedFixture };