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/r0/.github/CODEOWNERS

@@ -0,0 +1 @@
+* rust-embedded/cortex-a rust-embedded/cortex-m rust-embedded/cortex-r rust-embedded/msp430 rust-embedded/riscv

vendor/r0/.github/bors.toml

@@ -0,0 +1,4 @@
+block_labels = ["needs-decision"]
+delete_merged_branches = true
+required_approvals = 1
+status = ["continuous-integration/travis-ci/push"]