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/embassy-executor/build_common.rs

@@ -0,0 +1,126 @@
+// NOTE: this file is copy-pasted between several Embassy crates, because there is no
+// straightforward way to share this code:
+// - it cannot be placed into the root of the repo and linked from each build.rs using `#[path =
+// "../build_common.rs"]`, because `cargo publish` requires that all files published with a crate
+// reside in the crate's directory,
+// - it cannot be symlinked from `embassy-xxx/build_common.rs` to `../build_common.rs`, because
+// symlinks don't work on Windows.
+
+use std::collections::HashSet;
+use std::env;
+
+/// Helper for emitting cargo instruction for enabling configs (`cargo:rustc-cfg=X`) and declaring
+/// them (`cargo:rust-check-cfg=cfg(X)`).
+#[derive(Debug)]
+pub struct CfgSet {
+    enabled: HashSet<String>,
+    declared: HashSet<String>,
+}
+
+impl CfgSet {
+    pub fn new() -> Self {
+        Self {
+            enabled: HashSet::new(),
+            declared: HashSet::new(),
+        }
+    }
+
+    /// Enable a config, which can then be used in `#[cfg(...)]` for conditional compilation.
+    ///
+    /// All configs that can potentially be enabled should be unconditionally declared using
+    /// [`Self::declare()`].
+    pub fn enable(&mut self, cfg: impl AsRef<str>) {
+        if self.enabled.insert(cfg.as_ref().to_owned()) {
+            println!("cargo:rustc-cfg={}", cfg.as_ref());
+        }
+    }
+
+    pub fn enable_all(&mut self, cfgs: &[impl AsRef<str>]) {
+        for cfg in cfgs.iter() {
+            self.enable(cfg.as_ref());
+        }
+    }
+
+    /// Declare a valid config for conditional compilation, without enabling it.
+    ///
+    /// This enables rustc to check that the configs in `#[cfg(...)]` attributes are valid.
+    pub fn declare(&mut self, cfg: impl AsRef<str>) {
+        if self.declared.insert(cfg.as_ref().to_owned()) {
+            println!("cargo:rustc-check-cfg=cfg({})", cfg.as_ref());
+        }
+    }
+
+    pub fn declare_all(&mut self, cfgs: &[impl AsRef<str>]) {
+        for cfg in cfgs.iter() {
+            self.declare(cfg.as_ref());
+        }
+    }
+
+    pub fn set(&mut self, cfg: impl Into<String>, enable: bool) {
+        let cfg = cfg.into();
+        if enable {
+            self.enable(cfg.clone());
+        }
+        self.declare(cfg);
+    }
+}
+
+/// Sets configs that describe the target platform.
+pub fn set_target_cfgs(cfgs: &mut CfgSet) {
+    let target = env::var("TARGET").unwrap();
+
+    if target.starts_with("thumbv6m-") {
+        cfgs.enable_all(&["cortex_m", "armv6m"]);
+    } else if target.starts_with("thumbv7m-") {
+        cfgs.enable_all(&["cortex_m", "armv7m"]);
+    } else if target.starts_with("thumbv7em-") {
+        cfgs.enable_all(&["cortex_m", "armv7m", "armv7em"]);
+    } else if target.starts_with("thumbv8m.base") {
+        cfgs.enable_all(&["cortex_m", "armv8m", "armv8m_base"]);
+    } else if target.starts_with("thumbv8m.main") {
+        cfgs.enable_all(&["cortex_m", "armv8m", "armv8m_main"]);
+    }
+    cfgs.declare_all(&[
+        "cortex_m",
+        "armv6m",
+        "armv7m",
+        "armv7em",
+        "armv8m",
+        "armv8m_base",
+        "armv8m_main",
+    ]);
+
+    cfgs.set("has_fpu", target.ends_with("-eabihf"));
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub struct CompilerDate {
+    year: u16,
+    month: u8,
+    day: u8,
+}
+
+impl CompilerDate {
+    fn parse(date: &str) -> Option<Self> {
+        let mut parts = date.split('-');
+        let year = parts.next()?.parse().ok()?;
+        let month = parts.next()?.parse().ok()?;
+        let day = parts.next()?.parse().ok()?;
+        Some(Self { year, month, day })
+    }
+}
+
+impl PartialEq<&str> for CompilerDate {
+    fn eq(&self, other: &&str) -> bool {
+        let Some(other) = Self::parse(other) else {
+            return false;
+        };
+        self.eq(&other)
+    }
+}
+
+impl PartialOrd<&str> for CompilerDate {
+    fn partial_cmp(&self, other: &&str) -> Option<std::cmp::Ordering> {
+        Self::parse(other).map(|other| self.cmp(&other))
+    }
+}