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/litrs/src/test_util.rs

@@ -0,0 +1,113 @@
+use std::fmt::{Debug, Display};
+
+use crate::*;
+
+
+#[track_caller]
+pub(crate) fn assert_parse_ok_eq<T: PartialEq + Debug + Display>(
+    input: &str,
+    result: Result<T, ParseError>,
+    expected: T,
+    parse_method: &str,
+) {
+    match result {
+        Ok(actual) if actual == expected => {
+            if actual.to_string() != input {
+                panic!("formatting does not yield original input `{input}`: {actual:?}");
+            }
+        }
+        Ok(actual) => {
+            panic!("unexpected parsing result (with `{parse_method}`) for `{input}`:\n\
+                actual:    {actual:?}\n\
+                expected:  {expected:?}");
+        }
+        Err(e) => {
+            panic!("expected `{input}` to be parsed (with `{parse_method}`) successfully, \
+                but it failed: {e:?}");
+        }
+    }
+}
+
+// This is not ideal, but to perform this check we need `proc-macro2`. So we
+// just don't do anything if that feature is not enabled.
+#[cfg(not(feature = "proc-macro2"))]
+pub(crate) fn assert_roundtrip<T>(_: T, _: &str) {}
+
+#[cfg(feature = "proc-macro2")]
+#[track_caller]
+pub(crate) fn assert_roundtrip<T>(ours: T, input: &str)
+where
+    T: std::convert::TryFrom<proc_macro2::Literal> + fmt::Debug + PartialEq + Clone,
+    proc_macro2::Literal: From<T>,
+    <T as std::convert::TryFrom<proc_macro2::Literal>>::Error: std::fmt::Display,
+{
+    let pm_lit = input.parse::<proc_macro2::Literal>()
+        .expect("failed to parse input as proc_macro2::Literal");
+    let t_name = std::any::type_name::<T>();
+
+    // Unfortunately, `proc_macro2::Literal` does not implement `PartialEq`, so
+    // this is the next best thing.
+    if proc_macro2::Literal::from(ours.clone()).to_string() != pm_lit.to_string() {
+        panic!(
+            "Converting {} to proc_macro2::Literal has unexpected result:\
+                \nconverted: {:?}\nexpected:  {:?}",
+            t_name,
+            proc_macro2::Literal::from(ours),
+            pm_lit,
+        );
+    }
+
+    match T::try_from(pm_lit) {
+        Err(e) => {
+            panic!("Trying to convert proc_macro2::Literal to {t_name} results in error: {e}");
+        }
+        Ok(res) => {
+            if res != ours {
+                panic!("Converting proc_macro2::Literal to {t_name} has unexpected result:\n\
+                    actual:    {res:?}\n\
+                    expected:  {ours:?}");
+            }
+        }
+    }
+}
+
+macro_rules! assert_err {
+    ($ty:ident, $input:literal, $kind:ident, $( $span:tt )+ ) => {
+        assert_err_single!($ty::parse($input), $kind, $($span)+);
+        assert_err_single!($crate::Literal::parse($input), $kind, $($span)+);
+    };
+}
+
+macro_rules! assert_err_single {
+    ($expr:expr, $kind:ident, $( $span:tt )+ ) => {
+        let res = $expr;
+        let err = match res {
+            Err(e) => e,
+            Ok(v) => panic!(
+                "Expected `{}` to return an error, but it returned Ok({:?})",
+                stringify!($expr),
+                v,
+            ),
+        };
+        if err.kind != $crate::err::ParseErrorKind::$kind {
+            panic!(
+                "Expected error kind {} for `{}` but got {:?}",
+                stringify!($kind),
+                stringify!($expr),
+                err.kind,
+            )
+        }
+        let expected_span = assert_err_single!(@span $($span)+);
+        if err.span != expected_span {
+            panic!(
+                "Expected error span {:?} for `{}` but got {:?}",
+                expected_span,
+                stringify!($expr),
+                err.span,
+            )
+        }
+    };
+    (@span $start:literal .. $end:literal) => { Some($start .. $end) };
+    (@span $at:literal) => { Some($at.. $at + 1) };
+    (@span None) => { None };
+}