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/semihosting/src/panicking.rs

@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+#[allow(clippy::used_underscore_binding)]
+#[inline(never)]
+#[cfg_attr(not(test), panic_handler)]
+fn _panic(_info: &core::panic::PanicInfo<'_>) -> ! {
+    #[cfg(feature = "stdio")]
+    eprintln!("{_info}");
+
+    #[cfg(feature = "panic-unwind")]
+    {
+        use crate::atomic::Ordering;
+        // TODO(panic-unwind): PANICKED is global, so if panics occur concurrently on thread 1 and
+        // thread 2 this could be mistakenly thought to be a double panic. However, I'm not sure if
+        // there is a way to handle that well without thread local.
+        if crate::experimental::panic::PANICKED.fetch_add(1, Ordering::AcqRel) != 0 {
+            #[cfg(feature = "stdio")]
+            eprintln!("panic during panic, aborting");
+            crate::process::abort()
+        }
+    }
+
+    #[cfg(feature = "backtrace")]
+    {
+        use core::{ffi::c_void, ptr};
+
+        use unwinding::abi::{_Unwind_Backtrace, _Unwind_GetIP, UnwindContext, UnwindReasonCode};
+
+        extern "C" fn callback(
+            unwind_ctx: &UnwindContext<'_>,
+            _arg: *mut c_void,
+        ) -> UnwindReasonCode {
+            let ip = _Unwind_GetIP(unwind_ctx);
+            if ip == 0 {
+                UnwindReasonCode::NORMAL_STOP
+            } else {
+                eprintln!("  {ip:#x}");
+                UnwindReasonCode::NO_REASON
+            }
+        }
+
+        eprintln!("stack backtrace:");
+        _Unwind_Backtrace(callback, ptr::null_mut());
+    }
+
+    #[cfg(feature = "panic-unwind")]
+    {
+        let _code = unwinding::panic::begin_panic(alloc::boxed::Box::new(""));
+        #[cfg(feature = "stdio")]
+        eprintln!("failed to begin panic (unwind error code {})", _code.0);
+    }
+
+    crate::process::exit(101)
+}