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/anyhow/tests/ui/empty-ensure.stderr

@@ -0,0 +1,12 @@
+error: unexpected end of macro invocation
+ --> tests/ui/empty-ensure.rs:4:5
+  |
+4 |     ensure!();
+  |     ^^^^^^^^^ missing tokens in macro arguments
+  |
+note: while trying to match meta-variable `$cond:expr`
+ --> src/ensure.rs
+  |
+  |     ($cond:expr $(,)?) => {
+  |      ^^^^^^^^^^
+  = note: this error originates in the macro `$crate::__parse_ensure` which comes from the expansion of the macro `ensure` (in Nightly builds, run with -Z macro-backtrace for more info)