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/bitflags-1.3.2/tests/compile-pass/visibility/bits_field.rs

@@ -0,0 +1,11 @@
+use bitflags::bitflags;
+
+bitflags! {
+    pub struct Flags1: u32 {
+        const FLAG_A = 0b00000001;
+    }
+}
+
+fn main() {
+    assert_eq!(0b00000001, Flags1::FLAG_A.bits);
+}

vendor/bitflags-1.3.2/tests/compile-pass/visibility/pub_in.rs

@@ -0,0 +1,19 @@
+mod a {
+    mod b {
+        use bitflags::bitflags;
+
+        bitflags! {
+            pub(in crate::a) struct Flags: u32 {
+                const FLAG_A = 0b00000001;
+            }
+        }
+    }
+
+    pub fn flags() -> u32 {
+        b::Flags::FLAG_A.bits()
+    }
+}
+
+fn main() {
+    assert_eq!(0b00000001, a::flags());
+}