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/embedded-hal-0.2.7/src/digital/mod.rs

@@ -0,0 +1,27 @@
+//! Digital I/O
+//!
+//!
+//!
+
+// Deprecated / infallible traits
+#[deprecated(
+    since = "0.2.2",
+    note = "Deprecated because the methods cannot return errors. \
+                                      Users should use the traits in digital::v2."
+)]
+pub mod v1;
+
+// New / fallible traits
+pub mod v2;
+
+// v2 -> v1 compatibility wrappers
+// These require explicit casts from v2 -> v1
+pub mod v1_compat;
+
+// v1 -> v2 compatibility shims
+// These are implicit over v1 implementations
+pub mod v2_compat;
+
+// Re-export old traits so this isn't a breaking change
+#[allow(deprecated)]
+pub use self::v1::*;