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/ssd1306/release.toml

@@ -0,0 +1,8 @@
+pre-release-replacements = [
+  {file="CHANGELOG.md", search="[Uu]nreleased", replace="{{version}}"},
+  {file="CHANGELOG.md", search="\\.\\.\\.HEAD", replace="...{{tag_name}}"},
+  {file="CHANGELOG.md", search="ReleaseDate", replace="{{date}}"},
+  {file="CHANGELOG.md", search="<!-- next-header -->", replace="<!-- next-header -->\n\n## [Unreleased] - ReleaseDate"},
+  {file="CHANGELOG.md", search="<!-- next-url -->", replace="<!-- next-url -->\n[Unreleased]: https://github.com/rust-embedded-community/ssd1306/compare/{{tag_name}}...HEAD"},
+]
+tag-message = "Release {{crate_name}} {{version}}"