docs(install): document rustup mirror fallback

Co-authored-by: nsight <wuyefeima@163.com>

docs/INSTALL.md

@@ -109,6 +109,15 @@ export RUSTUP_UPDATE_ROOT=https://mirrors.tuna.tsinghua.edu.cn/rustup/rustup
 curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable
 ```
 
+If the TUNA mirror is slow from your network, `rsproxy.cn` is another
+rustup mirror option for Linux/macOS:
+
+```bash
+export RUSTUP_DIST_SERVER=https://rsproxy.cn
+export RUSTUP_UPDATE_ROOT=https://rsproxy.cn/rustup
+curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable
+```
+
 The `RUSTUP_DIST_SERVER` and `RUSTUP_UPDATE_ROOT` environment variables must
 be set **before** running rustup-init; the toolchain download otherwise hits
 the same TLS handshake problem as the installer.
@@ -346,6 +355,20 @@ Set `DEEPSEEK_TUI_RELEASE_BASE_URL` to a mirrored release-asset directory
 (rsproxy, TUNA, Tencent COS, Aliyun OSS), or skip npm entirely and use the
 Cargo mirror setup in [Section 3](#3-install-via-cargo-any-tier-1-rust-target).
 
+### Debian/Ubuntu: `feature edition2024 is required` from `cargo install`
+
+Some Debian/Ubuntu distro packages ship an older Cargo that cannot parse Rust
+2024 crates. For example, Cargo 1.75.0 fails before building with:
+
+```text
+feature `edition2024` is required
+```
+
+Install current stable Rust through rustup, then rerun the two Cargo install
+commands from [Section 3](#3-install-via-cargo-any-tier-1-rust-target). After
+rustup finishes, `which cargo` should point to `~/.cargo/bin/cargo`, not
+`/usr/bin/cargo`.
+
 ### Debian/Ubuntu: `error: linker 'cc' not found` while building
 
 Install the C toolchain: