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-macro2/src/marker.rs

@@ -0,0 +1,17 @@
+use alloc::rc::Rc;
+use core::marker::PhantomData;
+use core::panic::{RefUnwindSafe, UnwindSafe};
+
+// Zero sized marker with the correct set of autotrait impls we want all proc
+// macro types to have.
+#[derive(Copy, Clone)]
+#[cfg_attr(
+    all(procmacro2_semver_exempt, any(not(wrap_proc_macro), super_unstable)),
+    derive(PartialEq, Eq)
+)]
+pub(crate) struct ProcMacroAutoTraits(PhantomData<Rc<()>>);
+
+pub(crate) const MARKER: ProcMacroAutoTraits = ProcMacroAutoTraits(PhantomData);
+
+impl UnwindSafe for ProcMacroAutoTraits {}
+impl RefUnwindSafe for ProcMacroAutoTraits {}