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/cortex-m/build.rs

@@ -0,0 +1,54 @@
+use std::path::PathBuf;
+use std::{env, fs};
+
+fn main() {
+    let target = env::var("TARGET").unwrap();
+    let host_triple = env::var("HOST").unwrap();
+    let out_dir = PathBuf::from(env::var("OUT_DIR").unwrap());
+    let name = env::var("CARGO_PKG_NAME").unwrap();
+
+    if host_triple == target {
+        println!("cargo:rustc-cfg=native");
+    }
+
+    if target.starts_with("thumb") {
+        let suffix = if env::var_os("CARGO_FEATURE_LINKER_PLUGIN_LTO").is_some() {
+            "-lto"
+        } else {
+            ""
+        };
+
+        fs::copy(
+            format!("bin/{}{}.a", target, suffix),
+            out_dir.join(format!("lib{}.a", name)),
+        )
+        .unwrap();
+
+        println!("cargo:rustc-link-lib=static={}", name);
+        println!("cargo:rustc-link-search={}", out_dir.display());
+    }
+
+    if target.starts_with("thumbv6m-") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv6m");
+    } else if target.starts_with("thumbv7m-") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv7m");
+    } else if target.starts_with("thumbv7em-") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv7m");
+        println!("cargo:rustc-cfg=armv7em"); // (not currently used)
+    } else if target.starts_with("thumbv8m.base") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv8m");
+        println!("cargo:rustc-cfg=armv8m_base");
+    } else if target.starts_with("thumbv8m.main") {
+        println!("cargo:rustc-cfg=cortex_m");
+        println!("cargo:rustc-cfg=armv8m");
+        println!("cargo:rustc-cfg=armv8m_main");
+    }
+
+    if target.ends_with("-eabihf") {
+        println!("cargo:rustc-cfg=has_fpu");
+    }
+}