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/darling/tests/compile-fail/with_closure_capture.stderr

@@ -0,0 +1,30 @@
+error[E0308]: mismatched types
+  --> tests/compile-fail/with_closure_capture.rs:12:16
+   |
+12 |           with = |m| Ok(
+   |                  ^ arguments to this function are incorrect
+   |  ________________|
+   | |
+13 | |             String::from_meta(m)?.to_uppercase()
+14 | |             + example1.1.as_ref().map(|s| s.as_str()).unwrap_or("")
+15 | |         ),
+   | |_________^ expected fn pointer, found closure
+   |
+   = note: expected fn pointer `for<'a> fn(&'a syn::attr::Meta) -> Result<String, darling::Error>`
+                 found closure `{closure@$DIR/tests/compile-fail/with_closure_capture.rs:12:16: 12:19}`
+note: closures can only be coerced to `fn` types if they do not capture any variables
+  --> tests/compile-fail/with_closure_capture.rs:14:15
+   |
+14 |             + example1.1.as_ref().map(|s| s.as_str()).unwrap_or("")
+   |               ^^^^^^^^ `example1` captured here
+help: the return type of this call is `{closure@$DIR/tests/compile-fail/with_closure_capture.rs:12:16: 12:19}` due to the type of the argument passed
+  --> tests/compile-fail/with_closure_capture.rs:12:16
+   |
+12 |           with = |m| Ok(
+   |  ________________^
+13 | |             String::from_meta(m)?.to_uppercase()
+14 | |             + example1.1.as_ref().map(|s| s.as_str()).unwrap_or("")
+15 | |         ),
+   | |_________- this argument influences the return type of `{{root}}`
+note: function defined here
+  --> $RUST/core/src/convert/mod.rs