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/micromath/src/float/signum.rs

@@ -0,0 +1,33 @@
+//! Returns a number that represents the sign of `self`.
+
+use super::F32;
+
+impl F32 {
+    /// Returns a number that represents the sign of `self`.
+    ///
+    /// * `1.0` if the number is positive, `+0.0` or `INFINITY`
+    /// * `-1.0` if the number is negative, `-0.0` or `NEG_INFINITY`
+    /// * `NAN` if the number is `NAN`
+    pub fn signum(self) -> Self {
+        if self.is_nan() {
+            Self::NAN
+        } else {
+            F32(1.0).copysign(self)
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::F32;
+
+    #[test]
+    fn sanity_check() {
+        assert_eq!(F32::INFINITY.signum(), F32(1.0));
+        assert_eq!(F32(0.0).signum(), F32(1.0));
+        assert_eq!(F32(1.0).signum(), F32(1.0));
+        assert_eq!(F32::NEG_INFINITY.signum(), F32(-1.0));
+        assert_eq!(F32(-0.0).signum(), F32(-1.0));
+        assert_eq!(F32(-1.0).signum(), F32(-1.0));
+    }
+}