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/postcard/tests/crc.rs

@@ -0,0 +1,85 @@
+#[test]
+#[cfg(feature = "use-crc")]
+fn test_crc() {
+    use crc::{Crc, CRC_32_ISCSI};
+
+    let data: &[u8] = &[0x01, 0x00, 0x20, 0x30];
+    let buffer = &mut [0u8; 32];
+    let crc = Crc::<u32>::new(&CRC_32_ISCSI);
+    let digest = crc.digest();
+    let res = postcard::to_slice_crc32(data, buffer, digest).unwrap();
+    assert_eq!(res, &[0x04, 0x01, 0x00, 0x20, 0x30, 0x8E, 0xC8, 0x1A, 0x37]);
+
+    let digest = crc.digest();
+    let res = postcard::take_from_bytes_crc32::<[u8; 5]>(res, digest).unwrap();
+
+    let expected_bytes = [0x04, 0x01, 0x00, 0x20, 0x30];
+    let remaining_bytes = [];
+    assert_eq!(res, (expected_bytes, remaining_bytes.as_slice()));
+}
+
+#[test]
+#[cfg(feature = "use-crc")]
+fn test_crc_8() {
+    use crc::{Crc, CRC_8_SMBUS};
+
+    let data: &[u8] = &[0x01, 0x00, 0x20, 0x30];
+    let buffer = &mut [0u8; 32];
+    let crc = Crc::<u8>::new(&CRC_8_SMBUS);
+    let digest = crc.digest();
+    let res = postcard::ser_flavors::crc::to_slice_u8(data, buffer, digest).unwrap();
+    assert_eq!(res, &[0x04, 0x01, 0x00, 0x20, 0x30, 167]);
+
+    let digest = crc.digest();
+    let res = postcard::de_flavors::crc::take_from_bytes_u8::<[u8; 5]>(res, digest).unwrap();
+
+    let expected_bytes = [0x04, 0x01, 0x00, 0x20, 0x30];
+    let remaining_bytes = [];
+    assert_eq!(res, (expected_bytes, remaining_bytes.as_slice()));
+}
+
+#[test]
+#[cfg(feature = "use-crc")]
+fn test_crc_error() {
+    use crc::{Crc, CRC_32_ISCSI};
+
+    let data: &[u8] = &[0x01, 0x00, 0x20, 0x30];
+    let buffer = &mut [0u8; 32];
+    let crc = Crc::<u32>::new(&CRC_32_ISCSI);
+    let digest = crc.digest();
+    let res = postcard::to_slice_crc32(data, buffer, digest).unwrap();
+
+    // intentionally corrupt the crc
+    let last = res.len() - 1;
+    res[last] = 0;
+
+    let digest = crc.digest();
+    let res = postcard::take_from_bytes_crc32::<[u8; 5]>(res, digest);
+
+    assert_eq!(res, Err(postcard::Error::DeserializeBadCrc));
+}
+
+#[test]
+#[cfg(feature = "use-crc")]
+fn test_crc_in_method() {
+    use crc::{Crc, CRC_32_ISCSI};
+    use postcard::{to_slice_crc32, Result};
+    use serde::Serialize;
+
+    #[derive(Debug, Serialize)]
+    pub struct Thing {
+        value: u32,
+    }
+
+    impl Thing {
+        pub fn to_bytes<'a>(&self, buf: &'a mut [u8]) -> Result<&'a mut [u8]> {
+            let crc = Crc::<u32>::new(&CRC_32_ISCSI);
+            to_slice_crc32(self, buf, crc.digest())
+        }
+    }
+
+    let buffer = &mut [0u8; 5];
+    let thing = Thing { value: 42 };
+    let slice = thing.to_bytes(buffer).unwrap();
+    assert_eq!(slice, &[0x2A, 0xB7, 0xF5, 0x22, 0x19]);
+}