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/proc-macro-error/tests/runtime-errors.rs

@@ -0,0 +1,13 @@
+use proc_macro_error::*;
+
+#[test]
+#[should_panic = "proc-macro-error API cannot be used outside of"]
+fn missing_attr_emit() {
+    emit_call_site_error!("You won't see me");
+}
+
+#[test]
+#[should_panic = "proc-macro-error API cannot be used outside of"]
+fn missing_attr_abort() {
+    abort_call_site!("You won't see me");
+}