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/probe/proc_macro_span.rs

@@ -0,0 +1,55 @@
+// This code exercises the surface area that we expect of Span's unstable API.
+// If the current toolchain is able to compile it, then proc-macro2 is able to
+// offer these APIs too.
+
+#![cfg_attr(procmacro2_build_probe, no_std)]
+#![cfg_attr(procmacro2_build_probe, feature(proc_macro_span))]
+
+extern crate alloc;
+extern crate proc_macro;
+extern crate std;
+
+use alloc::string::String;
+use core::ops::{Range, RangeBounds};
+use proc_macro::{Literal, Span};
+use std::path::PathBuf;
+
+pub fn byte_range(this: &Span) -> Range<usize> {
+    this.byte_range()
+}
+
+pub fn start(this: &Span) -> Span {
+    this.start()
+}
+
+pub fn end(this: &Span) -> Span {
+    this.end()
+}
+
+pub fn line(this: &Span) -> usize {
+    this.line()
+}
+
+pub fn column(this: &Span) -> usize {
+    this.column()
+}
+
+pub fn file(this: &Span) -> String {
+    this.file()
+}
+
+pub fn local_file(this: &Span) -> Option<PathBuf> {
+    this.local_file()
+}
+
+pub fn join(this: &Span, other: Span) -> Option<Span> {
+    this.join(other)
+}
+
+pub fn subspan<R: RangeBounds<usize>>(this: &Literal, range: R) -> Option<Span> {
+    this.subspan(range)
+}
+
+// Include in sccache cache key.
+#[cfg(procmacro2_build_probe)]
+const _: Option<&str> = option_env!("RUSTC_BOOTSTRAP");

vendor/proc-macro2/src/probe/proc_macro_span_file.rs

@@ -0,0 +1,19 @@
+// The subset of Span's API stabilized in Rust 1.88.
+
+#![cfg_attr(procmacro2_build_probe, no_std)]
+
+extern crate alloc;
+extern crate proc_macro;
+extern crate std;
+
+use alloc::string::String;
+use proc_macro::Span;
+use std::path::PathBuf;
+
+pub fn file(this: &Span) -> String {
+    this.file()
+}
+
+pub fn local_file(this: &Span) -> Option<PathBuf> {
+    this.local_file()
+}

vendor/proc-macro2/src/probe/proc_macro_span_location.rs

@@ -0,0 +1,25 @@
+// The subset of Span's API stabilized in Rust 1.88.
+
+#![cfg_attr(procmacro2_build_probe, no_std)]
+
+extern crate alloc;
+extern crate proc_macro;
+extern crate std;
+
+use proc_macro::Span;
+
+pub fn start(this: &Span) -> Span {
+    this.start()
+}
+
+pub fn end(this: &Span) -> Span {
+    this.end()
+}
+
+pub fn line(this: &Span) -> usize {
+    this.line()
+}
+
+pub fn column(this: &Span) -> usize {
+    this.column()
+}