docs(tests): explain model preview dependencies and loading completion

tests/headless/model-loader.test.mjs

@@ -48,6 +48,7 @@ test('Host shares concurrent model reads per identity/workspace but never reuses
     const abort = new AbortController(), a = host.loadModel(account, workspace, root, { signal: abort.signal, navigationGeneration: 1 }), b = host.loadModel(account, workspace, root, { navigationGeneration: 2 });
     await tick(); const rejected = assert.rejects(a, { code: 'load_aborted' }); abort.abort(); await rejected; assert.equal(calls[0].signal.aborted, false);
     gate.resolve(); assert.equal((await b).versions.get(root), 1); assert.equal(calls.length, 1); assert.equal(factories, 1);
+    // 两个订阅均已结束，空闲 canonical 结果不能充当下一次权威读取；新调用应看到版本 2。
     version = 2; assert.equal((await host.loadModel(account, workspace, root)).versions.get(root), 2); assert.equal(calls.length, 2); assert.equal(host.getWorkspaces().size, 0);
   } finally { gate.resolve(); await host.close(); }
 });
@@ -86,6 +87,7 @@ test('Host previews share one source but cancelled observers never receive a lat
   yield part(3,{status:'ready',document:(await import('../../src/lib/canvas-core/document.ts')).createDocument(root,canvases,[first,second]),versions:new Map([[root,1]])},true);
  };}});
  const abort=new AbortController();const a=host.loadModel(account,workspace,root,{signal:abort.signal,onPreview:value=>aStates.push(value)}),b=host.loadModel(account,workspace,root,{onPreview:value=>bStates.push(value)});
+ // 等首个元素真正进入预览再取消 A；记录 A 的通知数，放行 B 的后缀后它必须保持不变。
  try{while(!aStates.some(value=>value.receivedElements===1))await tick();assert.equal(reads,1);assert.equal(aStates.at(-1).document.elements.size,1);const rejected=assert.rejects(a,{code:'load_aborted'});abort.abort();await rejected;const count=aStates.length;gate.resolve();const complete=await b;assert.equal(complete.document.elements.size,2);assert.equal(aStates.length,count);assert.equal(bStates.at(-1).document.elements.size,2);assert.equal(host.getWorkspaces().size,0);}finally{gate.resolve();await host.close();}
 });
 // onPreview 抛错经 onObserverError 隔离，不阻止 canonical ready；身份退役后不能继续通知第二次读取的预览。