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/semver-parser/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "semver-parser"
+version = "0.7.0"
+authors = ["Steve Klabnik <steve@steveklabnik.com>"]
+license = "MIT/Apache-2.0"
+repository = "https://github.com/steveklabnik/semver-parser"
+homepage = "https://github.com/steveklabnik/semver-parser"
+documentation = "https://docs.rs/semver-parser"
+description = """
+Parsing of the semver spec.
+"""