book: cover remaining C++26 proposals in preview chapter

Add a 12.2 subsection summarizing the remaining tracked C++26 proposals:
structured binding enhancements, trivially relocatable types,
user-generated static_assert messages / constexpr structured bindings,
std::submdspan and span/mdspan enhancements, hardened library, and
debugging support. Refs #318

Author: changkun

book/en-us/12-cpp26.md

@@ -105,6 +105,19 @@ C++26 also plans to bring many new library facilities, including:
 
 > Most of these are not yet implemented in current standard libraries; whether a few of them (such as `std::hive`) are ultimately included in C++26 remains uncertain — defer to the final standard and cppreference.
 
+### Other notable proposals
+
+Beyond the features above, C++26 brings many improvements to both the language and the library, for example:
+
+- **Structured binding enhancements**: allowing structured bindings in conditions (`if`/`while`), and introducing packs in structured bindings;
+- **Trivially relocatable types**: letting "move then destroy" be optimized into a single memory copy;
+- **User-generated `static_assert` messages** and **`constexpr` structured bindings**: making compile-time assertion messages producible from constant expressions;
+- **`std::submdspan`** and other enhancements to `std::span` / `std::mdspan` (such as construction from an `initializer_list`);
+- a **hardened standard library** and `std::is_within_lifetime`, improving safety;
+- **debugging support**, such as `std::breakpoint` and `std::is_debugger_present`.
+
+> Again, the final form and inclusion of these proposals are subject to the final standard.
+
 ## 12.3 On the standard and implementation status
 
 To reiterate: C++26 has not been officially released, the **final syntax and inclusion of the features described here may change**, and only a small subset (pack indexing, `= delete` with a reason, the placeholder `_`, saturation arithmetic) can currently be compiled and run. Readers are encouraged to follow [cppreference: C++26](https://en.cppreference.com/w/cpp/26) and the [compiler support page](https://en.cppreference.com/w/cpp/compiler_support) for the latest progress.

book/zh-cn/12-cpp26.md

@@ -105,6 +105,19 @@ C++26 还计划带来许多新的库设施，包括：
 
 > 这些设施在当前标准库中大多尚未实现；其中个别设施（如 `std::hive`）是否会最终纳入 C++26 仍存在不确定性，请以最终标准与 cppreference 为准。
 
+### 其他值得关注的提案
+
+除上述特性外，C++26 还在语言与库两方面带来许多改进，例如：
+
+- **结构化绑定的增强**：允许结构化绑定出现在 `if`/`while` 等条件中，以及在结构化绑定中引入包（pack）；
+- **平凡可重定位类型**（trivially relocatable）：让「移动后再销毁」可以被优化为一次内存拷贝；
+- **用户生成的 `static_assert` 消息** 与 **`constexpr` 结构化绑定**：使编译期断言信息可由常量表达式生成；
+- **`std::submdspan`** 等对 `std::span` / `std::mdspan` 的增强（如从 `initializer_list` 构造）；
+- **加固标准库**（hardened library）与 `std::is_within_lifetime`，提升安全性；
+- **调试支持**，如 `std::breakpoint`、`std::is_debugger_present`。
+
+> 同样地，这些提案的最终形态与纳入与否仍以最终标准为准。
+
 ## 12.3 关于标准与实现状态
 
 再次强调：C++26 尚未正式发布，本章所述特性的**最终语法与是否纳入标准都可能发生变化**，且当前能够编译运行的仅是其中一小部分（如形参包索引、带原因的 `= delete`、占位符 `_`、饱和算术）。建议读者持续关注 [cppreference: C++26](https://en.cppreference.com/w/cpp/26) 与 [编译器支持情况](https://en.cppreference.com/w/cpp/compiler_support) 以获取最新进展。