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/esp-hal/src/debugger.rs

@@ -0,0 +1,25 @@
+//! Debugger utilities
+
+/// Checks if a debugger is connected.
+pub fn debugger_connected() -> bool {
+    #[cfg(xtensa)]
+    {
+        xtensa_lx::is_debugger_attached()
+    }
+
+    #[cfg(riscv)]
+    {
+        use crate::peripherals::ASSIST_DEBUG;
+        let assist_debug = unsafe { &*ASSIST_DEBUG::ptr() };
+        assist_debug
+            .core_0_debug_mode()
+            .read()
+            .core_0_debug_module_active()
+            .bit_is_set()
+    }
+
+    #[cfg(not(any(xtensa, riscv)))]
+    {
+        false
+    }
+}