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-0.20.11/tests/compile-fail/flatten_meta_conflicts.stderr

@@ -0,0 +1,17 @@
+error: `flatten` and `multiple` cannot be used together
+  --> tests/compile-fail/flatten_meta_conflicts.rs:11:24
+   |
+11 |     #[darling(flatten, multiple, with = demo, skip = true)]
+   |                        ^^^^^^^^
+
+error: `flatten` and `with` cannot be used together
+  --> tests/compile-fail/flatten_meta_conflicts.rs:11:34
+   |
+11 |     #[darling(flatten, multiple, with = demo, skip = true)]
+   |                                  ^^^^
+
+error: `flatten` and `skip` cannot be used together
+  --> tests/compile-fail/flatten_meta_conflicts.rs:11:47
+   |
+11 |     #[darling(flatten, multiple, with = demo, skip = true)]
+   |                                               ^^^^