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/rustversion/tests/test_const.rs

@@ -0,0 +1,42 @@
+#![allow(
+    clippy::semicolon_if_nothing_returned, // https://github.com/rust-lang/rust-clippy/issues/7324
+    clippy::used_underscore_items,
+)]
+
+#[rustversion::attr(all(), const)]
+fn _basic() {}
+const _BASIC: () = _basic();
+
+#[rustversion::attr(all(), const)]
+unsafe fn _unsafe() {}
+const _UNSAFE: () = unsafe { _unsafe() };
+
+macro_rules! item {
+    ($i:item) => {
+        #[rustversion::attr(all(), const)]
+        $i
+    };
+}
+
+item! {fn _item() {}}
+const _ITEM: () = _item();
+
+macro_rules! ident {
+    ($fn:ident) => {
+        #[rustversion::attr(all(), const)]
+        $fn _ident() {}
+    };
+}
+
+ident! {fn}
+const _IDENT: () = _ident();
+
+#[rustversion::attr(all(), const)]
+/// doc
+fn _doc_below() {}
+const _DOC_BELOW: () = _doc_below();
+
+/// doc
+#[rustversion::attr(all(), const)]
+fn _doc_above() {}
+const _DOC_ABOVE: () = _doc_above();