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/memo-map/Makefile

@@ -0,0 +1,27 @@
+all: test
+
+build:
+	@cargo build --all-features
+
+doc:
+	@cargo doc --all-features
+
+test:
+	@cargo test
+
+miri-test:
+	@cargo +nightly miri test
+
+format:
+	@rustup component add rustfmt 2> /dev/null
+	@cargo fmt --all
+
+format-check:
+	@rustup component add rustfmt 2> /dev/null
+	@cargo fmt --all -- --check
+
+lint:
+	@rustup component add clippy 2> /dev/null
+	@cargo clippy
+
+.PHONY: all doc test format format-check lint