docs(tests): explain durable recovery cuts and replay ownership

tests/headless/durability.test.mjs

@@ -18,6 +18,8 @@ async function fixture(){let broken=false;const errors=[],engine=await openProje
   // 全部网络入口都故意抛错，保证测试只观察本地元数据而不是悄悄依赖网络同步。
   persistence:{compile(){throw Error('unexpected write');},async submit(){throw Error('unexpected write');},async query(){throw Error('unexpected query');},canonical(){throw Error('unexpected canonical');}},onError:error=>errors.push(error)});await engine.persist();return{engine,errors,fail:value=>broken=value};}
 // runtime 修改先通知脏修订，保存后通知对应持久化修订；关闭也通知进度，但完成保存不再增加一次业务 Root。关闭后订阅为空清理函数。
+// updateRuntime 即使只返回内容相同的新对象也会形成元数据根；revision 在 persist 前捕获，用于排除保存过程额外发布根。
 test('durability subscribers observe dirty metadata, final durable cut and closure without another business Root',async()=>{const{engine}=await fixture(),states=[];const stop=engine.subscribeDurability(()=>states.push(engine.getDurability()));assert.strictEqual(engine.getDurability(),engine.getDurability());engine.editor.updateRuntime(previous=>({...previous}));const revision=engine.getSnapshot().rootRevision;assert.ok(states.some(state=>state.rootRevision>state.durableRootRevision));await engine.persist();assert.equal(states.at(-1).durableRootRevision,revision);assert.equal(engine.getSnapshot().rootRevision,revision);await engine.close();assert.ok(states.some(state=>state.closing&&!state.closed));assert.equal(states.at(-1).closed,true);stop();const noop=engine.subscribeDurability(()=>assert.fail('closed owner notified'));noop();});
 // 一个监听器抛错不妨碍其他监听器看到 quota；数字 DOM 风格错误归一成字符串，保存失败保持脏状态，重试成功后清除错误。
+// broken 只让内存 commit 抛错，不制造真实磁盘配额不足；errors 数组单独核对观察者错误的隔离通路。
 test('storage failures remain observable and normalize numeric DOM-style codes without listener exceptions changing durability',async()=>{const f=await fixture();const states=[];f.engine.subscribeDurability(()=>{throw Error('observer');});f.engine.subscribeDurability(()=>states.push(f.engine.getDurability()));f.fail(true);f.engine.editor.updateRuntime(previous=>({...previous}));await assert.rejects(f.engine.persist());assert.equal(states.at(-1).error,'quota');assert.equal(typeof states.at(-1).error,'string');assert.ok(states.at(-1).durableRootRevision<states.at(-1).rootRevision);f.fail(false);await f.engine.persist();assert.equal(states.at(-1).error,undefined);assert.equal(states.at(-1).rootRevision,states.at(-1).durableRootRevision);assert.ok(f.errors.some(error=>error.message==='observer'));await f.engine.close();});