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/paste/tests/ui/invalid-ident.stderr

@@ -0,0 +1,26 @@
+error: expected identifier, found `0f`
+ --> tests/ui/invalid-ident.rs:3:1
+  |
+3 | / paste! {
+4 | |     fn [<0 f>]() {}
+5 | | }
+  | |_^ expected identifier
+  |
+help: identifiers cannot start with a number
+ --> tests/ui/invalid-ident.rs:3:1
+  |
+3 | paste! {
+  | ^
+  = note: this error originates in the macro `paste` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: `"f\""` is not a valid identifier
+ --> tests/ui/invalid-ident.rs:8:8
+  |
+8 |     fn [<f '"'>]() {}
+  |        ^^^^^^^^^
+
+error: `"f'"` is not a valid identifier
+  --> tests/ui/invalid-ident.rs:12:8
+   |
+12 |     fn [<f "'">]() {}
+   |        ^^^^^^^^^