ESP32-S3 摄像头图传：固件 + Slint 客户端 + 共享协议

两块 ESP32-S3-WROOM-1-N16R8 图传板（DVP 接口接 OV2640/OV3660/OV5640），
向队伍客户端与裁判终端推 UDP 图传。固件是跑在 ESP-IDF 上的 Rust，用官方
esp32-camera 组件采集；客户端是 Rust + Slint 桌面程序。

三个 crate 各自独立构建：
  .            firmware，交叉编译到 Xtensa
  protocol/    线路格式与帧重组，两端共用，无 ESP-IDF 依赖，有单元测试
  client/      Slint 桌面客户端

视频走 UDP 而非 TCP：实测传感器产出 22 fps，TCP MJPEG 只送出 1.9 fps，
且只占用链路能力的 1%——链路是空的，帧堵在队头阻塞后面等一个实时画面
等不起的重传计时器。改为独立数据报、不重传、缺片丢整帧后达到 20.8 fps。

发现是双向的：板子每 2 秒广播 Hello，客户端广播 Subscribe，板子据此定向
推流。板子身份由 MAC 派生，重启不变，所以客户端可以把绑定存进配置。

裁判端同时收两块板子（每个发送方一个重组器），队伍端绑定其中一块并只显示
自己那一边的血量。装甲板接口已预留（ArmorSource trait + ARMOR_PORT），
当前无数据源，界面显示占位符。

vendor 了 esp32-camera 2.1.7：组件注册表的版本求解器找不到它，且需要打一处
补丁（见 components/esp32-camera/target/esp32s3/ll_cam.c）。

.gitignore 三条 target/ 都锚定在根目录，因为 protocol/ 与 client/ 各自也是
独立 crate；而 components/esp32-camera/target/ 是上游的按芯片分目录，里面是
源码，必须保留。

client/Cargo.toml

@@ -0,0 +1,29 @@
+[package]
+name = "cam-client"
+version = "0.1.0"
+edition = "2021"
+
+[build-dependencies]
+# Compiles ui/camera.slint into Rust before the crate itself builds. Kept separate
+# from [dependencies] because it only runs on the build host, never on the target.
+slint-build = "1"
+
+[dependencies]
+# Shared wire format with the firmware. Kept as a path dependency rather than
+# copied, so the two sides cannot drift apart silently.
+protocol = { path = "../protocol" }
+
+# Slint for the UI. The default renderer on macOS is femtovg, which needs no
+# extra system libraries; `backend-winit` provides the window integration.
+slint = "1"
+
+# JPEG decoding for display. Slint's Image can take raw RGBA, so the JPEG has to
+# be decoded somewhere - `image` does that, and nothing on this side re-encodes.
+image = { version = "0.25", default-features = false, features = ["jpeg"] }
+
+anyhow = "1"
+log = "0.4"
+env_logger = "0.11"
+
+[profile.release]
+opt-level = 2