feat: add display UI with splash screen, dashboard, and RSSI sparkline

Implement full SSD1306 OLED display support:
- Splash screen on boot (1.5s) with project name, version, board
- Dashboard showing radio state, config, counters, RSSI/SNR
- RSSI history sparkline (64-sample ring buffer, 2px-wide bars)
- DisplayOn/DisplayOff host commands routed via dedicated channel
- Pure render functions against generic DrawTarget (no hardware coupling)
- Display on/off control bypasses radio task entirely

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/riscv/build.rs

@@ -0,0 +1,17 @@
+use std::env;
+
+fn main() {
+    println!("cargo:rustc-check-cfg=cfg(riscv)");
+    println!("cargo:rustc-check-cfg=cfg(riscv32)");
+    println!("cargo:rustc-check-cfg=cfg(riscv64)");
+
+    let target_arch = env::var("CARGO_CFG_TARGET_ARCH").unwrap();
+
+    if target_arch == "riscv32" {
+        println!("cargo:rustc-cfg=riscv");
+        println!("cargo:rustc-cfg=riscv32");
+    } else if target_arch == "riscv64" {
+        println!("cargo:rustc-cfg=riscv");
+        println!("cargo:rustc-cfg=riscv64");
+    }
+}