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/max_size.rs

@@ -0,0 +1,101 @@
+#![allow(unused_imports)]
+
+#[cfg(feature = "experimental-derive")]
+mod tests {
+    use postcard::experimental::max_size::MaxSize;
+    use postcard::to_slice;
+    use serde::Serialize;
+
+    #[test]
+    fn test_struct_max_size() {
+        #[derive(MaxSize)]
+        struct Foo {
+            _a: u16,
+            _b: Option<u8>,
+        }
+
+        assert_eq!(Foo::POSTCARD_MAX_SIZE, 5);
+    }
+
+    #[test]
+    fn test_enum_max_size() {
+        #[allow(dead_code)]
+        #[derive(MaxSize, Serialize)]
+        enum Bar {
+            A(u16),
+            B(u8),
+        }
+
+        assert_eq!(Bar::POSTCARD_MAX_SIZE, 4);
+        let mut buf = [0u8; 128];
+        let used = to_slice(&Bar::A(0xFFFF), &mut buf).unwrap();
+        assert!(
+            used.len() <= Bar::POSTCARD_MAX_SIZE,
+            "FAIL {} > {}",
+            used.len(),
+            Bar::POSTCARD_MAX_SIZE
+        );
+
+        #[derive(MaxSize)]
+        enum Baz {}
+
+        assert_eq!(Baz::POSTCARD_MAX_SIZE, 0);
+    }
+
+    #[test]
+    fn test_ref() {
+        #[allow(dead_code)]
+        #[derive(MaxSize)]
+        struct Foo {
+            a: &'static u32,
+        }
+    }
+
+    #[cfg(feature = "heapless")]
+    #[test]
+    fn test_vec_edge_cases() {
+        #[track_caller]
+        fn test_equals<const N: usize>(buf: &mut [u8]) {
+            let mut v = heapless::Vec::<u8, N>::new();
+            for _ in 0..N {
+                v.push(0).unwrap();
+            }
+
+            let serialized = postcard::to_slice(&v, buf).unwrap();
+
+            assert_eq!(heapless::Vec::<u8, N>::POSTCARD_MAX_SIZE, serialized.len());
+        }
+
+        let mut buf = [0; 16400];
+
+        test_equals::<1>(&mut buf);
+        test_equals::<2>(&mut buf);
+
+        test_equals::<127>(&mut buf);
+        test_equals::<128>(&mut buf);
+        test_equals::<129>(&mut buf);
+
+        test_equals::<16383>(&mut buf);
+        test_equals::<16384>(&mut buf);
+        test_equals::<16385>(&mut buf);
+    }
+
+    // #[cfg(feature = "experimental-derive")]
+    // #[test]
+    // fn test_union_max_size() {
+    //     #[derive(postcard::MaxSize)]
+    //     union Foo {
+    //         a: u16,
+    //         b: Option<u8>,
+    //     }
+    // }
+
+    // #[cfg(feature = "experimental-derive")]
+    // #[test]
+    // fn test_not_implemented() {
+    //     #[derive(postcard::MaxSize)]
+    //     struct Foo {
+    //         a: &'static str,
+    //     }
+    // }
+}