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/lora-modulation/src/lib.rs

@@ -0,0 +1,266 @@
+#![deny(rust_2018_idioms)]
+#![cfg_attr(not(test), no_std)]
+#![doc = include_str!("../README.md")]
+
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+/// Channel width. Lower values increase time on air, but may be able to find clear frequencies.
+pub enum Bandwidth {
+    _7KHz,
+    _10KHz,
+    _15KHz,
+    _20KHz,
+    _31KHz,
+    _41KHz,
+    _62KHz,
+    _125KHz,
+    _250KHz,
+    _500KHz,
+}
+
+impl Bandwidth {
+    pub const fn hz(self) -> u32 {
+        match self {
+            Bandwidth::_7KHz => 7810u32,
+            Bandwidth::_10KHz => 10420u32,
+            Bandwidth::_15KHz => 15630u32,
+            Bandwidth::_20KHz => 20830u32,
+            Bandwidth::_31KHz => 31250u32,
+            Bandwidth::_41KHz => 41670u32,
+            Bandwidth::_62KHz => 62500u32,
+            Bandwidth::_125KHz => 125000u32,
+            Bandwidth::_250KHz => 250000u32,
+            Bandwidth::_500KHz => 500000u32,
+        }
+    }
+}
+
+impl From<Bandwidth> for u32 {
+    fn from(value: Bandwidth) -> Self {
+        value.hz()
+    }
+}
+
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+/// Controls the chirp rate. Lower values are slower bandwidth (longer time on air), but more robust.
+pub enum SpreadingFactor {
+    _5,
+    _6,
+    _7,
+    _8,
+    _9,
+    _10,
+    _11,
+    _12,
+}
+
+impl SpreadingFactor {
+    pub const fn factor(self) -> u32 {
+        match self {
+            SpreadingFactor::_5 => 5,
+            SpreadingFactor::_6 => 6,
+            SpreadingFactor::_7 => 7,
+            SpreadingFactor::_8 => 8,
+            SpreadingFactor::_9 => 9,
+            SpreadingFactor::_10 => 10,
+            SpreadingFactor::_11 => 11,
+            SpreadingFactor::_12 => 12,
+        }
+    }
+}
+
+impl From<SpreadingFactor> for u32 {
+    fn from(sf: SpreadingFactor) -> Self {
+        sf.factor()
+    }
+}
+
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+/// Controls the forward error correction. Higher values are more robust, but reduces the ratio
+/// of actual data in transmissions.
+pub enum CodingRate {
+    _4_5,
+    _4_6,
+    _4_7,
+    _4_8,
+}
+
+impl CodingRate {
+    pub const fn denom(&self) -> u32 {
+        match self {
+            CodingRate::_4_5 => 5,
+            CodingRate::_4_6 => 6,
+            CodingRate::_4_7 => 7,
+            CodingRate::_4_8 => 8,
+        }
+    }
+}
+
+/// LoRa modulation parameters barring frequency
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct BaseBandModulationParams {
+    pub sf: SpreadingFactor,
+    pub bw: Bandwidth,
+    pub cr: CodingRate,
+    /// low data rate optimization, see SX127x datasheet section 4.1.1.6
+    pub ldro: bool,
+    /// eagerly pre-calculated symbol duration in microseconds
+    t_sym_us: u32,
+}
+
+impl BaseBandModulationParams {
+    /// Create a set of parameters, possible forcing low data rate optimization on or off.
+    /// Low data rate optimization is determined automatically
+    /// based on `sf` and `bw` according to Semtech's datasheets for SX126x/SX127x
+    /// (enabled if symbol length is >= 16.38ms)
+    pub const fn new(sf: SpreadingFactor, bw: Bandwidth, cr: CodingRate) -> Self {
+        let t_sym_us = 2u32.pow(sf.factor()) * 1_000_000 / bw.hz();
+        // according to SX127x 4.1.1.6 it's 16ms
+        // SX126x says it's 16.38ms
+        // probably it's 16.384ms which is SF11@125kHz
+        let ldro = t_sym_us >= 16_384;
+        Self { sf, bw, cr, ldro, t_sym_us }
+    }
+
+    pub const fn delay_in_symbols(&self, delay_in_ms: u32) -> u16 {
+        (delay_in_ms * 1000 / self.t_sym_us) as u16
+    }
+
+    pub const fn symbols_to_ms(&self, symbols: u32) -> u32 {
+        (self.t_sym_us * symbols) / 1_000
+    }
+
+    /// Calculates time on air for a given payload and modulation parameters.
+    /// If `preamble` is None, the whole preamble including syncword is excluded from calculation.
+    pub const fn time_on_air_us(
+        &self,
+        preamble: Option<u8>,
+        explicit_header: bool,
+        len: u8,
+    ) -> u32 {
+        let sf = self.sf.factor() as i32;
+        let t_sym_us = self.t_sym_us;
+
+        let cr = self.cr.denom() as i32;
+        let de = if self.ldro {
+            1
+        } else {
+            0
+        };
+        let h = if explicit_header {
+            0
+        } else {
+            1
+        };
+
+        const fn div_ceil(num: i32, denom: i32) -> i32 {
+            (num - 1) / denom + 1
+        }
+
+        let big_ratio = div_ceil(8 * len as i32 - 4 * sf + 28 + 16 - 20 * h, 4 * (sf - 2 * de));
+        let big_ratio = if big_ratio > 0 {
+            big_ratio
+        } else {
+            0
+        };
+        let payload_symb_nb = (8 + big_ratio * cr) as u32;
+
+        match preamble {
+            None => t_sym_us * payload_symb_nb,
+            Some(preamble) => (4 * preamble as u32 + 17 + 4 * payload_symb_nb) * t_sym_us / 4,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const LORAWAN_OVERHEAD: u8 = 13;
+    // the shortest t_sym
+    const SF5BW500: BaseBandModulationParams =
+        BaseBandModulationParams::new(SpreadingFactor::_5, Bandwidth::_500KHz, CodingRate::_4_5);
+
+    // EU868 DR6
+    const SF7BW250: BaseBandModulationParams =
+        BaseBandModulationParams::new(SpreadingFactor::_7, Bandwidth::_250KHz, CodingRate::_4_5);
+    // EU868 DR5
+    const SF7BW125: BaseBandModulationParams =
+        BaseBandModulationParams::new(SpreadingFactor::_7, Bandwidth::_125KHz, CodingRate::_4_5);
+    // EU868 DR4
+    const SF8BW125: BaseBandModulationParams =
+        BaseBandModulationParams::new(SpreadingFactor::_8, Bandwidth::_125KHz, CodingRate::_4_5);
+    // EU868 DR3
+    const SF9BW125: BaseBandModulationParams =
+        BaseBandModulationParams::new(SpreadingFactor::_9, Bandwidth::_125KHz, CodingRate::_4_5);
+    // EU868 DR2
+    const SF10BW125: BaseBandModulationParams =
+        BaseBandModulationParams::new(SpreadingFactor::_10, Bandwidth::_125KHz, CodingRate::_4_5);
+    // EU868 DR1
+    const SF11BW125: BaseBandModulationParams =
+        BaseBandModulationParams::new(SpreadingFactor::_11, Bandwidth::_125KHz, CodingRate::_4_5);
+    // EU868 DR0
+    const SF12BW125: BaseBandModulationParams =
+        BaseBandModulationParams::new(SpreadingFactor::_12, Bandwidth::_125KHz, CodingRate::_4_5);
+
+    fn lorawan_airtime_us(params: &BaseBandModulationParams, app_payload_length: u8) -> u32 {
+        params.time_on_air_us(Some(8), true, LORAWAN_OVERHEAD + app_payload_length)
+    }
+
+    // data for time-on-air tests is verified against:
+    // * https://www.thethingsnetwork.org/airtime-calculator
+    // * https://avbentem.github.io/airtime-calculator/ttn/
+
+    #[test]
+    fn time_on_air_for_short_messages() {
+        assert_eq!(1152, SF5BW500.time_on_air_us(None, true, 0));
+        assert_eq!(6656, SF7BW250.time_on_air_us(None, true, 0));
+        assert_eq!(13312, SF7BW125.time_on_air_us(None, true, 0));
+    }
+
+    #[test]
+    fn time_on_air() {
+        let length = 25;
+        assert_eq!(41_088, lorawan_airtime_us(&SF7BW250, length));
+        assert_eq!(82_176, lorawan_airtime_us(&SF7BW125, length));
+        assert_eq!(143_872, lorawan_airtime_us(&SF8BW125, length));
+        assert_eq!(267_264, lorawan_airtime_us(&SF9BW125, length));
+        assert_eq!(493_568, lorawan_airtime_us(&SF10BW125, length));
+        assert_eq!(1_069_056, lorawan_airtime_us(&SF11BW125, length));
+        assert_eq!(1_974_272, lorawan_airtime_us(&SF12BW125, length));
+
+        let length = 26;
+        assert_eq!(41_088, lorawan_airtime_us(&SF7BW250, length));
+        assert_eq!(82_176, lorawan_airtime_us(&SF7BW125, length));
+        assert_eq!(154_112, lorawan_airtime_us(&SF8BW125, length));
+        assert_eq!(267_264, lorawan_airtime_us(&SF9BW125, length));
+        assert_eq!(493_568, lorawan_airtime_us(&SF10BW125, length));
+        assert_eq!(1_069_056, lorawan_airtime_us(&SF11BW125, length));
+        assert_eq!(1_974_272, lorawan_airtime_us(&SF12BW125, length));
+
+        let length = 27;
+        assert_eq!(41_088, lorawan_airtime_us(&SF7BW250, length));
+        assert_eq!(82_176, lorawan_airtime_us(&SF7BW125, length));
+        assert_eq!(154_112, lorawan_airtime_us(&SF8BW125, length));
+        assert_eq!(287_744, lorawan_airtime_us(&SF9BW125, length));
+        assert_eq!(534_528, lorawan_airtime_us(&SF10BW125, length));
+        assert_eq!(1_069_056, lorawan_airtime_us(&SF11BW125, length));
+        assert_eq!(1_974_272, lorawan_airtime_us(&SF12BW125, length));
+
+        let length = 28;
+        assert_eq!(43_648, lorawan_airtime_us(&SF7BW250, length));
+        assert_eq!(87_296, lorawan_airtime_us(&SF7BW125, length));
+        assert_eq!(154_112, lorawan_airtime_us(&SF8BW125, length));
+        assert_eq!(287_744, lorawan_airtime_us(&SF9BW125, length));
+        assert_eq!(534_528, lorawan_airtime_us(&SF10BW125, length));
+        assert_eq!(1_150_976, lorawan_airtime_us(&SF11BW125, length));
+        assert_eq!(2_138_112, lorawan_airtime_us(&SF12BW125, length));
+    }
+}