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/zerocopy-derive/tests/ui/privacy.stable.stderr

@@ -0,0 +1,32 @@
+warning: variants `A` and `B` are never constructed
+  --> $DIR/privacy.rs:80:9
+   |
+79 |     pub enum Enum {
+   |              ---- variants in this enum
+80 |         A(u8, u16),
+   |         ^
+81 |         B { a: u8, b: u16 },
+   |         ^
+   |
+   = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default
+
+error: type `private::_::_::_::ẕb` is private
+   --> $DIR/privacy.rs:110:9
+    |
+110 |         _,
+    |         ^ private type
+
+error: type `private::_::_::_::ẕ1` is private
+   --> $DIR/privacy.rs:126:9
+    |
+126 |         _,
+    |         ^ private type
+
+error: type `private::_::_::_::ẕb` is private
+   --> $DIR/privacy.rs:138:49
+    |
+138 |     let _: <Union as zerocopy_renamed::HasField<_, _, { zerocopy_renamed::ident_id!(b) }>>::Type =
+    |                                                 ^ private type
+
+error: aborting due to 3 previous errors; 1 warning emitted
+