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/tests/ui/misuse.stderr

@@ -0,0 +1,24 @@
+error[E0599]: the method `FIRST_ARG_MUST_EITHER_BE_Span_OR_IMPLEMENT_ToTokens_OR_BE_SpanRange` exists for reference `&Foo`, but its trait bounds were not satisfied
+ --> tests/ui/misuse.rs:7:5
+  |
+3 | struct Foo;
+  | ---------- doesn't satisfy `Foo: quote::to_tokens::ToTokens`
+...
+7 |     abort!(Foo, "BOOM");
+  |     ^^^^^^^^^^^^^^^^^^^ method cannot be called on `&Foo` due to unsatisfied trait bounds
+  |
+  = note: the following trait bounds were not satisfied:
+          `Foo: quote::to_tokens::ToTokens`
+          which is required by `&Foo: ToTokensAsSpanRange`
+note: the trait `quote::to_tokens::ToTokens` must be implemented
+ --> $CARGO/quote-1.0.37/src/to_tokens.rs
+  |
+  | pub trait ToTokens {
+  | ^^^^^^^^^^^^^^^^^^
+  = help: items from traits can only be used if the trait is implemented and in scope
+  = note: the following traits define an item `FIRST_ARG_MUST_EITHER_BE_Span_OR_IMPLEMENT_ToTokens_OR_BE_SpanRange`, perhaps you need to implement one of them:
+          candidate #1: `Span2AsSpanRange`
+          candidate #2: `SpanAsSpanRange`
+          candidate #3: `SpanRangeAsSpanRange`
+          candidate #4: `ToTokensAsSpanRange`
+  = note: this error originates in the macro `$crate::diagnostic` which comes from the expansion of the macro `abort` (in Nightly builds, run with -Z macro-backtrace for more info)