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/toml_datetime/src/ser.rs

@@ -0,0 +1,242 @@
+//! Serialization support for [`Datetime`][crate::Datetime]
+
+/// Check if serializing a [`Datetime`][crate::Datetime]
+pub fn is_datetime(name: &'static str) -> bool {
+    crate::datetime::is_datetime(name)
+}
+
+/// See [`DatetimeSerializer`]
+#[derive(Debug)]
+#[non_exhaustive]
+pub enum SerializerError {
+    /// Unsupported datetime format
+    InvalidFormat(crate::DatetimeParseError),
+    /// Unsupported serialization protocol
+    InvalidProtocol,
+}
+
+impl serde_core::ser::Error for SerializerError {
+    fn custom<T>(_msg: T) -> Self
+    where
+        T: core::fmt::Display,
+    {
+        Self::InvalidProtocol
+    }
+}
+
+impl core::fmt::Display for SerializerError {
+    fn fmt(&self, formatter: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        match self {
+            Self::InvalidFormat(e) => e.fmt(formatter),
+            Self::InvalidProtocol => "invalid serialization protocol".fmt(formatter),
+        }
+    }
+}
+
+impl core::error::Error for SerializerError {}
+
+/// Serializer / format support for emitting [`Datetime`][crate::Datetime]
+#[derive(Default)]
+pub struct DatetimeSerializer {
+    value: Option<crate::Datetime>,
+}
+
+impl DatetimeSerializer {
+    /// Create a serializer to emit [`Datetime`][crate::Datetime]
+    pub fn new() -> Self {
+        Self { value: None }
+    }
+
+    /// See [`serde_core::ser::SerializeStruct::serialize_field`]
+    pub fn serialize_field<T>(
+        &mut self,
+        key: &'static str,
+        value: &T,
+    ) -> Result<(), SerializerError>
+    where
+        T: serde_core::ser::Serialize + ?Sized,
+    {
+        if key == crate::datetime::FIELD {
+            self.value = Some(value.serialize(DatetimeFieldSerializer::default())?);
+        }
+
+        Ok(())
+    }
+
+    /// See [`serde_core::ser::SerializeStruct::end`]
+    pub fn end(self) -> Result<crate::Datetime, SerializerError> {
+        self.value.ok_or(SerializerError::InvalidProtocol)
+    }
+}
+
+#[derive(Default)]
+struct DatetimeFieldSerializer {}
+
+impl serde_core::ser::Serializer for DatetimeFieldSerializer {
+    type Ok = crate::Datetime;
+    type Error = SerializerError;
+    type SerializeSeq = serde_core::ser::Impossible<Self::Ok, Self::Error>;
+    type SerializeTuple = serde_core::ser::Impossible<Self::Ok, Self::Error>;
+    type SerializeTupleStruct = serde_core::ser::Impossible<Self::Ok, Self::Error>;
+    type SerializeTupleVariant = serde_core::ser::Impossible<Self::Ok, Self::Error>;
+    type SerializeMap = serde_core::ser::Impossible<Self::Ok, Self::Error>;
+    type SerializeStruct = serde_core::ser::Impossible<Self::Ok, Self::Error>;
+    type SerializeStructVariant = serde_core::ser::Impossible<Self::Ok, Self::Error>;
+
+    fn serialize_bool(self, _value: bool) -> Result<Self::Ok, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_i8(self, _value: i8) -> Result<Self::Ok, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_i16(self, _value: i16) -> Result<Self::Ok, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_i32(self, _value: i32) -> Result<Self::Ok, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_i64(self, _value: i64) -> Result<Self::Ok, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_u8(self, _value: u8) -> Result<Self::Ok, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_u16(self, _value: u16) -> Result<Self::Ok, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_u32(self, _value: u32) -> Result<Self::Ok, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_u64(self, _value: u64) -> Result<Self::Ok, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_f32(self, _value: f32) -> Result<Self::Ok, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_f64(self, _value: f64) -> Result<Self::Ok, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_char(self, _value: char) -> Result<Self::Ok, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_str(self, v: &str) -> Result<Self::Ok, Self::Error> {
+        v.parse::<crate::Datetime>()
+            .map_err(SerializerError::InvalidFormat)
+    }
+
+    fn serialize_bytes(self, _value: &[u8]) -> Result<Self::Ok, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_none(self) -> Result<Self::Ok, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_some<T>(self, _value: &T) -> Result<Self::Ok, Self::Error>
+    where
+        T: serde_core::ser::Serialize + ?Sized,
+    {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_unit(self) -> Result<Self::Ok, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_unit_struct(self, _name: &'static str) -> Result<Self::Ok, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_unit_variant(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        _variant: &'static str,
+    ) -> Result<Self::Ok, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_newtype_struct<T>(
+        self,
+        _name: &'static str,
+        _value: &T,
+    ) -> Result<Self::Ok, Self::Error>
+    where
+        T: serde_core::ser::Serialize + ?Sized,
+    {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_newtype_variant<T>(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        _variant: &'static str,
+        _value: &T,
+    ) -> Result<Self::Ok, Self::Error>
+    where
+        T: serde_core::ser::Serialize + ?Sized,
+    {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_seq(self, _len: Option<usize>) -> Result<Self::SerializeSeq, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_tuple(self, _len: usize) -> Result<Self::SerializeTuple, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_tuple_struct(
+        self,
+        _name: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeTupleStruct, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_tuple_variant(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        _variant: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeTupleVariant, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_map(self, _len: Option<usize>) -> Result<Self::SerializeMap, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_struct(
+        self,
+        _name: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeStruct, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+
+    fn serialize_struct_variant(
+        self,
+        _name: &'static str,
+        _variant_index: u32,
+        _variant: &'static str,
+        _len: usize,
+    ) -> Result<Self::SerializeStructVariant, Self::Error> {
+        Err(SerializerError::InvalidProtocol)
+    }
+}