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/src/tests/bitflags_match.rs

@@ -0,0 +1,93 @@
+bitflags! {
+    #[derive(PartialEq)]
+    struct Flags: u8 {
+        const A = 1 << 0;
+        const B = 1 << 1;
+        const C = 1 << 2;
+        const D = 1 << 3;
+    }
+}
+
+fn flag_to_string(flag: Flags) -> String {
+    bitflags_match!(flag, {
+        Flags::A => "A".to_string(),
+        Flags::B => { "B".to_string() }
+        Flags::C => "C".to_string(),
+        Flags::D => "D".to_string(),
+        Flags::A | Flags::B => "A or B".to_string(),
+        Flags::A & Flags::B => { "A and B | empty".to_string() },
+        Flags::A ^ Flags::B => "A xor B".to_string(),
+        Flags::A | Flags::B | Flags::C => "A or B or C".to_string(),
+        Flags::A & Flags::B & Flags::C => "A and B and C".to_string(),
+        Flags::A ^ Flags::B ^ Flags::C => "A xor B xor C".to_string(),
+        Flags::A | Flags::B | Flags::C | Flags::D => "All flags".to_string(),
+        _ => "Unknown combination".to_string()
+    })
+}
+
+#[test]
+fn test_single_flags() {
+    assert_eq!(flag_to_string(Flags::A), "A");
+    assert_eq!(flag_to_string(Flags::B), "B");
+    assert_eq!(flag_to_string(Flags::C), "C");
+    assert_eq!(flag_to_string(Flags::D), "D");
+}
+
+#[test]
+fn test_or_operations() {
+    assert_eq!(flag_to_string(Flags::A | Flags::B), "A or B");
+    assert_eq!(
+        flag_to_string(Flags::A | Flags::B | Flags::C),
+        "A or B or C"
+    );
+    assert_eq!(
+        flag_to_string(Flags::A | Flags::B | Flags::C | Flags::D),
+        "All flags"
+    );
+}
+
+#[test]
+fn test_and_operations() {
+    assert_eq!(flag_to_string(Flags::A & Flags::A), "A");
+    assert_eq!(flag_to_string(Flags::A & Flags::B), "A and B | empty");
+    assert_eq!(
+        flag_to_string(Flags::A & Flags::B & Flags::C),
+        "A and B | empty"
+    ); // Since A, B, and C are mutually exclusive, the result of A & B & C is 0 ==> A & B & C = 0000 (i.e., empty).
+       // However, in the bitflags_match! statement (actually is if {..} else if {..} .. else {..}),
+       // the "A & B = 0000" condition is listed first, so 0000 will match "A & B" first,
+       // resulting in the output of the "A and B | empty" branch.
+    assert_eq!(
+        flag_to_string(Flags::A & Flags::B & Flags::C & Flags::D),
+        "A and B | empty"
+    );
+}
+
+#[test]
+fn test_xor_operations() {
+    assert_eq!(flag_to_string(Flags::A ^ Flags::B), "A or B"); // A | B = A ^ B == 0011
+    assert_eq!(flag_to_string(Flags::A ^ Flags::A), "A and B | empty");
+    assert_eq!(
+        flag_to_string(Flags::A ^ Flags::B ^ Flags::C),
+        "A or B or C"
+    );
+}
+
+#[test]
+fn test_complex_operations() {
+    assert_eq!(flag_to_string(Flags::A | (Flags::B & Flags::C)), "A");
+    assert_eq!(
+        flag_to_string((Flags::A | Flags::B) & (Flags::B | Flags::C)),
+        "B"
+    );
+    assert_eq!(
+        flag_to_string(Flags::A ^ (Flags::B | Flags::C)),
+        "A or B or C"
+    );
+}
+
+#[test]
+fn test_empty_and_full_flags() {
+    assert_eq!(flag_to_string(Flags::empty()), "A and B | empty");
+    assert_eq!(flag_to_string(Flags::all()), "All flags");
+}