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/snake_case.rs

@@ -0,0 +1,69 @@
+#![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(Debug, EnumAsInner)]
+#[allow(non_camel_case_types)]
+#[allow(clippy::upper_case_acronyms)]
+enum MixedCaseVariants {
+    XMLIsNotCool,
+    Rust_IsCoolThough(u32),
+    YMCA { named: i16 },
+}
+
+#[test]
+fn test_xml_unit() {
+    let mixed = MixedCaseVariants::XMLIsNotCool;
+
+    assert!(mixed.is_xml_is_not_cool());
+    assert!(mixed.as_rust_is_cool_though().is_none());
+    assert!(mixed.as_ymca().is_none());
+}
+
+#[test]
+fn test_rust_unnamed() {
+    let mixed = MixedCaseVariants::Rust_IsCoolThough(42);
+
+    assert!(!mixed.is_xml_is_not_cool());
+    assert!(mixed.as_rust_is_cool_though().is_some());
+    assert!(mixed.as_ymca().is_none());
+
+    assert_eq!(*mixed.as_rust_is_cool_though().unwrap(), 42);
+    assert_eq!(mixed.into_rust_is_cool_though().unwrap(), 42);
+}
+
+#[test]
+fn test_ymca_named() {
+    let mixed = MixedCaseVariants::YMCA { named: -32_768 };
+
+    assert!(!mixed.is_xml_is_not_cool());
+    assert!(mixed.as_rust_is_cool_though().is_none());
+    assert!(mixed.as_ymca().is_some());
+
+    assert_eq!(*mixed.as_ymca().unwrap(), (-32_768));
+    assert_eq!(mixed.into_ymca().unwrap(), (-32_768));
+}