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/zerocopy/benches/formats/coco_dynamic_padding.rs

@@ -0,0 +1,24 @@
+use zerocopy_derive::*;
+
+// The only valid value of this type are the bytes `0xC0C0`.
+#[derive(TryFromBytes, KnownLayout, Immutable)]
+#[repr(u16)]
+pub enum C0C0 {
+    _XC0C0 = 0xC0C0,
+}
+
+#[derive(FromBytes, KnownLayout, Immutable, SplitAt)]
+#[repr(C, align(4))]
+pub struct Packet<Magic> {
+    magic_number: Magic,
+    milk: u8,
+    mug_size: u8,
+    temperature: [u8; 5],
+    marshmallows: [[u8; 3]],
+}
+
+/// A packet begining with the magic number `0xC0C0`.
+pub type CocoPacket = Packet<C0C0>;
+
+/// A packet beginning with any two initialized bytes.
+pub type LocoPacket = Packet<[u8; 2]>;

vendor/zerocopy/benches/formats/coco_dynamic_size.rs

@@ -0,0 +1,27 @@
+use zerocopy_derive::*;
+
+// The only valid value of this type are the bytes `0xC0C0`.
+#[derive(TryFromBytes, KnownLayout, Immutable, IntoBytes)]
+#[repr(u16)]
+pub enum C0C0 {
+    _XC0C0 = 0xC0C0,
+}
+
+macro_rules! define_packet {
+    ($name: ident, $trait: ident, $leading_field: ty) => {
+        #[derive($trait, KnownLayout, Immutable, IntoBytes, SplitAt)]
+        #[repr(C, align(2))]
+        pub struct $name {
+            magic_number: $leading_field,
+            mug_size: u8,
+            temperature: u8,
+            marshmallows: [[u8; 2]],
+        }
+    };
+}
+
+/// Packet begins with bytes 0xC0C0.
+define_packet!(CocoPacket, TryFromBytes, C0C0);
+
+/// Packet begins with any two bytes.
+define_packet!(LocoPacket, FromBytes, [u8; 2]);

vendor/zerocopy/benches/formats/coco_static_size.rs

@@ -0,0 +1,27 @@
+use zerocopy_derive::*;
+
+// The only valid value of this type are the bytes `0xC0C0`.
+#[derive(TryFromBytes, KnownLayout, Immutable, IntoBytes)]
+#[repr(u16)]
+pub enum C0C0 {
+    _XC0C0 = 0xC0C0,
+}
+
+macro_rules! define_packet {
+    ($name: ident, $trait: ident, $leading_field: ty) => {
+        #[derive($trait, KnownLayout, Immutable, IntoBytes)]
+        #[repr(C, align(2))]
+        pub struct $name {
+            magic_number: $leading_field,
+            mug_size: u8,
+            temperature: u8,
+            marshmallows: [u8; 2],
+        }
+    };
+}
+
+/// Packet begins with bytes 0xC0C0.
+define_packet!(CocoPacket, TryFromBytes, C0C0);
+
+/// Packet begins with any two bytes.
+define_packet!(LocoPacket, FromBytes, [u8; 2]);