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/src/varint.rs

@@ -0,0 +1,103 @@
+/// Returns the maximum number of bytes required to encode T.
+pub const fn varint_max<T: Sized>() -> usize {
+    const BITS_PER_BYTE: usize = 8;
+    const BITS_PER_VARINT_BYTE: usize = 7;
+
+    // How many data bits do we need for this type?
+    let bits = core::mem::size_of::<T>() * BITS_PER_BYTE;
+
+    // We add (BITS_PER_VARINT_BYTE - 1), to ensure any integer divisions
+    // with a remainder will always add exactly one full byte, but
+    // an evenly divided number of bits will be the same
+    let roundup_bits = bits + (BITS_PER_VARINT_BYTE - 1);
+
+    // Apply division, using normal "round down" integer division
+    roundup_bits / BITS_PER_VARINT_BYTE
+}
+
+/// Returns the maximum value stored in the last encoded byte.
+pub const fn max_of_last_byte<T: Sized>() -> u8 {
+    let max_bits = core::mem::size_of::<T>() * 8;
+    let extra_bits = max_bits % 7;
+    (1 << extra_bits) - 1
+}
+
+#[inline]
+pub fn varint_usize(n: usize, out: &mut [u8; varint_max::<usize>()]) -> &mut [u8] {
+    let mut value = n;
+    for i in 0..varint_max::<usize>() {
+        out[i] = value.to_le_bytes()[0];
+        if value < 128 {
+            return &mut out[..=i];
+        }
+
+        out[i] |= 0x80;
+        value >>= 7;
+    }
+    debug_assert_eq!(value, 0);
+    &mut out[..]
+}
+
+#[inline]
+pub fn varint_u16(n: u16, out: &mut [u8; varint_max::<u16>()]) -> &mut [u8] {
+    let mut value = n;
+    for i in 0..varint_max::<u16>() {
+        out[i] = value.to_le_bytes()[0];
+        if value < 128 {
+            return &mut out[..=i];
+        }
+
+        out[i] |= 0x80;
+        value >>= 7;
+    }
+    debug_assert_eq!(value, 0);
+    &mut out[..]
+}
+
+#[inline]
+pub fn varint_u32(n: u32, out: &mut [u8; varint_max::<u32>()]) -> &mut [u8] {
+    let mut value = n;
+    for i in 0..varint_max::<u32>() {
+        out[i] = value.to_le_bytes()[0];
+        if value < 128 {
+            return &mut out[..=i];
+        }
+
+        out[i] |= 0x80;
+        value >>= 7;
+    }
+    debug_assert_eq!(value, 0);
+    &mut out[..]
+}
+
+#[inline]
+pub fn varint_u64(n: u64, out: &mut [u8; varint_max::<u64>()]) -> &mut [u8] {
+    let mut value = n;
+    for i in 0..varint_max::<u64>() {
+        out[i] = value.to_le_bytes()[0];
+        if value < 128 {
+            return &mut out[..=i];
+        }
+
+        out[i] |= 0x80;
+        value >>= 7;
+    }
+    debug_assert_eq!(value, 0);
+    &mut out[..]
+}
+
+#[inline]
+pub fn varint_u128(n: u128, out: &mut [u8; varint_max::<u128>()]) -> &mut [u8] {
+    let mut value = n;
+    for i in 0..varint_max::<u128>() {
+        out[i] = value.to_le_bytes()[0];
+        if value < 128 {
+            return &mut out[..=i];
+        }
+
+        out[i] |= 0x80;
+        value >>= 7;
+    }
+    debug_assert_eq!(value, 0);
+    &mut out[..]
+}