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-error2/src/imp/delegate.rs

@@ -0,0 +1,68 @@
+//! This implementation uses [`proc_macro::Diagnostic`], nightly only.
+
+use std::cell::Cell;
+
+use proc_macro::{Diagnostic as PDiag, Level as PLevel};
+
+use crate::{
+    abort_now, check_correctness,
+    diagnostic::{Diagnostic, Level, SuggestionKind},
+};
+
+pub fn abort_if_dirty() {
+    check_correctness();
+    if IS_DIRTY.with(|c| c.get()) {
+        abort_now()
+    }
+}
+
+pub(crate) fn cleanup() -> Vec<Diagnostic> {
+    IS_DIRTY.with(|c| c.set(false));
+    vec![]
+}
+
+pub(crate) fn emit_diagnostic(diag: Diagnostic) {
+    let Diagnostic {
+        level,
+        span_range,
+        msg,
+        suggestions,
+        children,
+    } = diag;
+
+    let span = span_range.collapse().unwrap();
+
+    let level = match level {
+        Level::Warning => PLevel::Warning,
+        Level::Error => {
+            IS_DIRTY.with(|c| c.set(true));
+            PLevel::Error
+        }
+    };
+
+    let mut res = PDiag::spanned(span, level, msg);
+
+    for (kind, msg, span) in suggestions {
+        res = match (kind, span) {
+            (SuggestionKind::Note, Some(span_range)) => {
+                res.span_note(span_range.collapse().unwrap(), msg)
+            }
+            (SuggestionKind::Help, Some(span_range)) => {
+                res.span_help(span_range.collapse().unwrap(), msg)
+            }
+            (SuggestionKind::Note, None) => res.note(msg),
+            (SuggestionKind::Help, None) => res.help(msg),
+        }
+    }
+
+    for (span_range, msg) in children {
+        let span = span_range.collapse().unwrap();
+        res = res.span_error(span, msg);
+    }
+
+    res.emit()
+}
+
+thread_local! {
+    static IS_DIRTY: Cell<bool> = Cell::new(false);
+}

vendor/proc-macro-error2/src/imp/fallback.rs

@@ -0,0 +1,30 @@
+//! This implementation uses self-written stable facilities.
+
+use crate::{
+    abort_now, check_correctness,
+    diagnostic::{Diagnostic, Level},
+};
+use std::cell::RefCell;
+
+pub fn abort_if_dirty() {
+    check_correctness();
+    ERR_STORAGE.with(|storage| {
+        if !storage.borrow().is_empty() {
+            abort_now()
+        }
+    });
+}
+
+pub(crate) fn cleanup() -> Vec<Diagnostic> {
+    ERR_STORAGE.with(|storage| storage.replace(Vec::new()))
+}
+
+pub(crate) fn emit_diagnostic(diag: Diagnostic) {
+    if diag.level == Level::Error {
+        ERR_STORAGE.with(|storage| storage.borrow_mut().push(diag));
+    }
+}
+
+thread_local! {
+    static ERR_STORAGE: RefCell<Vec<Diagnostic>> = const { RefCell::new(Vec::new()) };
+}