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/ahash/build.rs

@@ -0,0 +1,22 @@
+#![deny(warnings)]
+
+use std::env;
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rustc-check-cfg=cfg(specialize)");
+    if let Some(true) = version_check::supports_feature("specialize") {
+        println!("cargo:rustc-cfg=specialize");
+    }
+    let arch = env::var("CARGO_CFG_TARGET_ARCH").expect("CARGO_CFG_TARGET_ARCH was not set");
+    println!("cargo:rustc-check-cfg=cfg(folded_multiply)");
+    if arch.eq_ignore_ascii_case("x86_64")
+        || arch.eq_ignore_ascii_case("aarch64")
+        || arch.eq_ignore_ascii_case("mips64")
+        || arch.eq_ignore_ascii_case("powerpc64")
+        || arch.eq_ignore_ascii_case("riscv64gc")
+        || arch.eq_ignore_ascii_case("s390x")
+    {
+        println!("cargo:rustc-cfg=folded_multiply");
+    }
+}