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/bitfield/multitest.toml

@@ -0,0 +1,22 @@
+[[tests]]
+name = "cargo-test-{{toolchain}}"
+command = ["cargo", "+{{toolchain}}", "test", "--all", "--frozen"]
+
+[[tests.env]]
+name = "CARGO_TARGET_DIR"
+value = "target/{{name}}"
+
+[tests.variables]
+toolchain = ["stable", "beta", "nightly", "1.79.0"]
+
+[[tests]]
+name = "cargo-clippy"
+command = ["cargo", "+stable", "clippy", "--all", "--frozen", "--all-targets",  "--", "-D", "warnings"]
+
+[[tests.env]]
+name = "CARGO_TARGET_DIR"
+value = "target/cargo-test-nightly"
+
+[[tests]]
+name = "cargo-fmt"
+command = ["cargo", "fmt", "--all", "--", "--check"]