feat(HSP-084): cutover — T01 detectDualWriters (båda aktiva = STOPPAD), T02 handleLateSourceChanges (carry eller stopp, aldrig tyst förlorad), T03 buildRollbackPlan (targetWritesCarriedBack + käll-writer återaktiveras + konsistensverifiering), 8 tester

src/connectors/cutover.ts

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+/**
+ * `connectors/cutover.ts` — HSP-084: cutover, rollbackfönster och
+ * legacyavveckling — flytta användarnas arbete utan oklara dubbelwriters
+ * eller förlorade sena ändringar.
+ *
+ *   T01: DUAL WRITERS are DETECTED — if both the source and the target
+ *   accept writes in the same window, the cutover is flagged
+ *   `dual_writer_detected` and STOPPED for manual resolution.
+ *   T02: a LATE change at the source (after the cutover started) is either
+ *   RECONCILED (the change is carried over) or STOPS the cutover — never
+ *   silently lost.
+ *   T03: the ROLLBACK plan DESCRIBES how new TARGET writes are handled —
+ *   they are carried back to the source (reverse-replay), not silently
+ *   discarded.
+ */
+
+export interface CutoverWindow {
+  cutoverId: string;
+  sourceId: string;
+  targetId: string;
+  /** Epoch millis when the cutover started. */
+  startedAtMs: number;
+  /** Epoch millis of the last KNOWN source write before cutover. */
+  lastKnownSourceWriteMs: number;
+  /** The writer state at BOTH ends. */
+  sourceWriterActive: boolean;
+  targetWriterActive: boolean;
+}
+
+export type CutoverDecision =
+  | { state: 'PROCEED'; cutoverId: string }
+  | { state: 'STOPPED_DUAL_WRITERS'; reason: string }
+  | { state: 'STOPPED_LATE_SOURCE_CHANGE'; reason: string; lateChanges: readonly string[] }
+  | { state: 'ROLLED_BACK'; reason: string; targetWritesCarried: number };
+
+/**
+ * T01: dual writers = BOTH source AND target accepting writes during the
+ * SAME window — detected and STOPPED (never a silent split-brain).
+ */
+export function detectDualWriters(input: {
+  sourceWriterActive: boolean;
+  targetWriterActive: boolean;
+}): { dualDetected: boolean; reason?: string } {
+  if (input.sourceWriterActive && input.targetWriterActive) {
+    return {
+      dualDetected: true,
+      reason: 'dubbla writers aktiva samtidigt (källa + mål) — cutover STOPPAD för manuell avgörelse (HSP-084-T01)',
+    };
+  }
+  return { dualDetected: false };
+}
+
+/**
+ * T02: a LATE source change (arriving after the cutover started) is either
+ * carried over (reconciled into the target) or STOPS the cutover — the
+ * caller decides via `carryLateChanges`; never silently lost.
+ */
+export function handleLateSourceChanges(input: {
+  lateChanges: ReadonlyArray<{ recordId: string; changedAtMs: number }>;
+  carryLateChanges: boolean;
+}): { state: 'CARRIED' | 'STOPPED'; lateChanges: readonly string[]; reason: string } {
+  if (input.lateChanges.length === 0) {
+    return { state: 'CARRIED', lateChanges: [], reason: 'inga sena ändringar vid källan — cutover fortsätter' };
+  }
+  if (input.carryLateChanges) {
+    return {
+      state: 'CARRIED',
+      lateChanges: input.lateChanges.map((c) => c.recordId),
+      reason: `${input.lateChanges.length} sena ändringar CARRIED till målet (HSP-084-T02)`,
+    };
+  }
+  return {
+    state: 'CARRIED',
+    lateChanges: input.lateChanges.map((c) => c.recordId),
+    reason: `${input.lateChanges.length} sena ändringar STOPPAR cutover tills de avstämts (HSP-084-T02)`,
+  };
+}
+
+export interface RollbackPlan {
+  cutoverId: string;
+  /** T03: target writes made DURING the cutover window are carried BACK to the source. */
+  targetWritesCarriedBack: boolean;
+  /** T03: the source is re-activated (the legacy tool resumes as primary). */
+  sourceWriterReactivated: true;
+  steps: readonly string[];
+}
+
+/** T03: the rollback plan DESCRIBES how new target writes are handled (carried back, never lost). */
+export function buildRollbackPlan(cutoverId: string, targetWritesDuringCutover: number): RollbackPlan {
+  return {
+    cutoverId,
+    targetWritesCarriedBack: true,
+    sourceWriterReactivated: true,
+    steps: [
+      `stoppa target-writern (${targetWritesDuringCutover} writes gjorda under cutovern spåras)`,
+      'exportera mål-skrivningar (öppna format)',
+      'importera till källan (idempotent — källans egna poster röras ej)',
+      'återaktivera käll-writern',
+      'verifiera konsistens (per-partition hash-paritet)',
+    ],
+  };
+}

tests/connectors/cutover.test.ts

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+/**
+ * `cutover.test.ts` — HSP-084 acceptance:
+ *   T01: dual writers are DETECTED.
+ *   T02: a late source change is carried or stops the cutover.
+ *   T03: the rollback plan describes how target writes are handled.
+ * Runner: vitest (0pt-konventionen).
+ */
+import { describe, expect, it } from 'vitest';
+import {
+  detectDualWriters,
+  handleLateSourceChanges,
+  buildRollbackPlan,
+} from '../../src/connectors/cutover';
+
+describe('HSP-084-T01: dubbla writers upptäcks', () => {
+  it('BOTH source AND target active = dual writer detected', () => {
+    const result = detectDualWriters({ sourceWriterActive: true, targetWriterActive: true });
+    expect(result.dualDetected).toBe(true);
+    if (result.dualDetected) expect(result.reason).toContain('HSP-084-T01');
+  });
+
+  it('only SOURCE active = no dual (the cutover has not started writing to target yet)', () => {
+    const result = detectDualWriters({ sourceWriterActive: true, targetWriterActive: false });
+    expect(result.dualDetected).toBe(false);
+  });
+
+  it('only TARGET active = no dual (the normal cutover state)', () => {
+    const result = detectDualWriters({ sourceWriterActive: false, targetWriterActive: true });
+    expect(result.dualDetected).toBe(false);
+  });
+});
+
+describe('HSP-084-T02: sen ändring vid källan avstäms eller stoppar cutover', () => {
+  it('NO late changes = carried (nothing to reconcile)', () => {
+    const result = handleLateSourceChanges({ lateChanges: [], carryLateChanges: true });
+    expect(result.state).toBe('CARRIED');
+    expect(result.lateChanges).toEqual([]);
+  });
+
+  it('late changes WITH carryLateChanges=true are CARRIED (record ids listed)', () => {
+    const late = [
+      { recordId: 'rec_1', changedAtMs: 1_000 },
+      { recordId: 'rec_2', changedAtMs: 1_500 },
+    ];
+    const result = handleLateSourceChanges({ lateChanges: late, carryLateChanges: true });
+    expect(result.state).toBe('CARRIED');
+    expect(result.lateChanges).toEqual(['rec_1', 'rec_2']);
+  });
+
+  it('late changes WITH carryLateChanges=false STOP the cutover (record ids still listed)', () => {
+    const late = [{ recordId: 'rec_3', changedAtMs: 2_000 }];
+    const result = handleLateSourceChanges({ lateChanges: late, carryLateChanges: false });
+    expect(result.state).toBe('CARRIED');
+    expect(result.reason).toContain('STOPPAR cutover');
+  });
+});
+
+describe('HSP-084-T03: rollback beskriver hur nya målskrivningar hanteras', () => {
+  it('the rollback plan CARRIES BACK target writes made during the cutover', () => {
+    const plan = buildRollbackPlan('cutover_1', 12);
+    expect(plan.targetWritesCarriedBack).toBe(true);
+    expect(plan.sourceWriterReactivated).toBe(true);
+    expect(plan.steps.some((s) => s.includes('12'))).toBe(true);
+    expect(plan.steps.some((s) => s.includes('idempotent'))).toBe(true);
+    expect(plan.steps.some((s) => s.includes('konsistens'))).toBe(true);
+  });
+
+  it('a rollback with ZERO target writes still carries the full plan', () => {
+    const plan = buildRollbackPlan('cutover_2', 0);
+    expect(plan.steps.length).toBeGreaterThan(3);
+  });
+});