refactor(loader): tighten the disabled gate and rehome the note

Address review: the gate module docstring now states the actual evaluation
contexts (config after injections against the plugin context, disabled at
every mount decision against the loader context); metadataExpressionErrors
rejects expressions nested below disabled and syntax-checks the disabled
expression itself so an unparseable gate fails at the gate instead of the
boot. The tutorial's !!js claims follow, and the loader note moves to
implemented/architecture with its inbound links retargeted.

docs/cordis-tutorial/05-config.zh.md

@@ -77,7 +77,7 @@ ValidationError: invalid config:
     greeting: !!js process.env.DEMO_GREETING ?? 'Hello'
 ```
 
-`!!js` **仅在 `config` 内有效**。Cordis 配置项的元数据（`name`、`id`、`disabled`、`inject` 等）是静态的；`disabled: !!js ...` 会生成一个真值表达式对象，始终禁用该 Cordis 配置项。详见 [loader 配置](../cordis-primer.md#loader-configuration)。
+`!!js` 仅在 `config` 与条目 `disabled` 字段内有效。`disabled: !!js ...` 在每次挂载决策时基于 loader 上下文求值（本仓库的扩展），可以按平台或环境门控一行；其余元数据（`name`、`id`、`inject` 等）保持静态，其中的表达式是普通真值数据。详见 [loader 配置](../cordis-primer.md#loader-configuration)。
 
 下一章：[组合与 HMR（热模块替换）](06-composition-and-hmr.md)：将 `cordis.yml` 视为应用。