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/az/etc/appveyor.yml

@@ -0,0 +1,35 @@
+# Copyright © 2019–2021 Trevor Spiteri
+
+# Copying and distribution of this file, with or without modification,
+# are permitted in any medium without royalty provided the copyright
+# notice and this notice are preserved. This file is offered as-is,
+# without any warranty.
+
+skip_tags: true
+
+environment:
+  matrix:
+    - TARGET: x86_64-pc-windows-msvc
+    - TARGET: x86_64-pc-windows-gnu
+    - TARGET: i686-pc-windows-msvc
+    - TARGET: i686-pc-windows-gnu
+
+install:
+  - set PATH=%USERPROFILE%\.cargo\bin;%PATH%
+  - curl -sSf -o rustup-init.exe https://win.rustup.rs
+  - rustup-init.exe -y --default-host %TARGET% --default-toolchain none --no-modify-path
+  - rustup --version
+  - rustup install beta-%TARGET% nightly-%TARGET% 1.31.1-%TARGET%
+  - rustup component add --toolchain beta-%TARGET% rustfmt clippy
+
+build: false
+
+test_script:
+  - cargo +beta-%TARGET% clippy --all-targets --features fail-on-warnings
+  - cargo +beta-%TARGET% test --features fail-on-warnings
+  - cargo +beta-%TARGET% test --release --features fail-on-warnings
+  - cargo +beta-%TARGET% fmt -- --check
+  - cargo +nightly-%TARGET% test --features fail-on-warnings
+  - cargo +nightly-%TARGET% test --release --features fail-on-warnings
+  - cargo +1.31.1-%TARGET% test --features fail-on-warnings
+  - cargo +1.31.1-%TARGET% test --release --features fail-on-warnings