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/enum-as-inner/tests/unit.rs

@@ -0,0 +1,61 @@
+#![warn(
+    clippy::default_trait_access,
+    clippy::dbg_macro,
+    clippy::print_stdout,
+    clippy::unimplemented,
+    clippy::use_self,
+    missing_copy_implementations,
+    missing_docs,
+    non_snake_case,
+    non_upper_case_globals,
+    rust_2018_idioms,
+    unreachable_pub
+)]
+
+use enum_as_inner::EnumAsInner;
+
+pub mod name_collisions {
+    #![allow(dead_code, missing_copy_implementations, missing_docs)]
+    pub struct Option;
+    pub struct Some;
+    pub struct None;
+    pub struct Result;
+    pub struct Ok;
+    pub struct Err;
+}
+#[allow(unused_imports)]
+use name_collisions::*;
+
+#[derive(EnumAsInner)]
+enum UnitVariants {
+    Zero,
+    One,
+    Two,
+}
+
+#[test]
+fn test_zero_unit() {
+    let unit = UnitVariants::Zero;
+
+    assert!(unit.is_zero());
+    assert!(!unit.is_one());
+    assert!(!unit.is_two());
+}
+
+#[test]
+fn test_one_unit() {
+    let unit = UnitVariants::One;
+
+    assert!(!unit.is_zero());
+    assert!(unit.is_one());
+    assert!(!unit.is_two());
+}
+
+#[test]
+fn test_two_unit() {
+    let unit = UnitVariants::Two;
+
+    assert!(!unit.is_zero());
+    assert!(!unit.is_one());
+    assert!(unit.is_two());
+}