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/zerocopy/tests/ui/transmute-ptr-to-usize.msrv.stderr

@@ -0,0 +1,19 @@
+error[E0277]: the trait bound `*const usize: IntoBytes` is not satisfied
+  --> $DIR/transmute-ptr-to-usize.rs:21:30
+   |
+21 | const POINTER_VALUE: usize = transmute!(&0usize as *const usize);
+   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `IntoBytes` is not implemented for `*const usize`
+   |
+note: required by a bound in `POINTER_VALUE::transmute`
+  --> $DIR/transmute-ptr-to-usize.rs:21:30
+   |
+21 | const POINTER_VALUE: usize = transmute!(&0usize as *const usize);
+   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                              |
+   |                              required by a bound in this
+   |                              required by this bound in `POINTER_VALUE::transmute`
+   = note: this error originates in the macro `transmute` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.