docs(tests): explain copy mappings and resumable planning checks

tests/headless/duplicate-scheduling.test.mjs

@@ -21,6 +21,7 @@ const selection=[{id:'F',incarnation:1}],options=()=>{let id=0;return{createId:(
 test('cooperative deep planning yields throughout scanning, allocation and remapping and matches the synchronous plan',async()=>{
  let slices=0;const plan=await planDuplicateAsync(document,registry,selection,options(),{now:()=>0,maxStepsPerSlice:64,yieldTask:async()=>{slices++;}});
  assert.ok(slices>200);assert.deepEqual(plan,planDuplicate(document,registry,selection,options()));assert.equal(validJson(plan),true);
+ // 规划只生成 changes；显式 applyChanges 后再验证文档，并检查复制入边仍从原上级 C 出发。
  const after=applyChanges(document,plan.changes);validateDocument(after,registry);
  assert.equal(after.elements.get(plan.elementIds.edge).source.elementId,'C');assert.equal(after.elements.get(plan.elementIds.edge).target.elementId,plan.elementIds.F);
  assert.equal(Object.isFrozen(plan.changes[0].after.data),true);assert.equal(document.elements.size,2003);
@@ -75,6 +76,7 @@ test('mapping output preserves special source IDs as own JSON fields without pro
 
 // 对比每次保存与间隔保存：最终映射、所有 yield 和阶段覆盖一致，而全量检查点显著减少，最大步距仍受限。
 test('stream checkpoint cadence reduces full snapshots while preserving every task yield, phase and final mapping',async()=>{
+ // now 固定为 0，所以 50ms 间隔不会自行到期；阶段切换和步数上限仍会保存，yield 次数必须相同。
  const binding={sourceToken:'captured',schemaToken:'schema'},run=async interval=>{let yields=0;const checkpoints=[],state=await prepareDuplicateStream(document,registry,selection,options(),binding,{now:()=>0,maxStepsPerSlice:64,checkpointIntervalMs:interval,maxStepsPerCheckpoint:2048,yieldTask:async()=>{yields++;},checkpoint:async state=>{checkpoints.push(state);}});return{state,yields,checkpoints};},every=await run(0),paced=await run(50);
  assert.deepEqual(paced.state,every.state);assert.equal(paced.yields,every.yields);assert.ok(paced.checkpoints.length<every.checkpoints.length/4);const phases=new Set(paced.checkpoints.map(state=>state.phase));for(const state of every.checkpoints)assert.ok(phases.has(state.phase));assert.equal(paced.checkpoints.at(-1).phase,'emit');assert.ok(paced.checkpoints.every(Object.isFrozen));for(let i=1;i<paced.checkpoints.length;i++)assert.ok(paced.checkpoints[i].steps-paced.checkpoints[i-1].steps<=2048+64);assert.equal(document.elements.size,2003);
 });