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/encode_unicode/tests/exhaustive.rs

@@ -0,0 +1,34 @@
+/* Copyright 2018 The encode_unicode Developers
+ *
+ * Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
+ * http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
+ * http://opensource.org/licenses/MIT>, at your option. This file may not be
+ * copied, modified, or distributed except according to those terms.
+ */
+
+//! Tests that try all possible values for at least one parameter / byte / unit
+//! of the tested function.
+
+use std::char;
+extern crate encode_unicode;
+use encode_unicode::*;
+
+#[test]
+fn from_ascii() {
+    for cp in 0u32..256 {
+        assert_eq!(Utf8Char::from_ascii(cp as u8).is_ok(), cp & 0x80 == 0);
+        if let Ok(u8c) = Utf8Char::from_ascii(cp as u8) {
+            assert_eq!(u8c, Utf8Char::from(cp as u8 as char));
+        }
+    }
+}
+
+#[test]
+fn from_bmp() {
+    for cp in 0u32..0x1_00_00 {
+        assert_eq!(
+            Utf16Char::from_bmp(cp as u16).ok(),
+            char::from_u32(cp).map(|u32c| Utf16Char::from(u32c) )
+        );
+    }
+}