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/ssmarshal/.gitlab-ci.yml

@@ -0,0 +1,26 @@
+build:
+    image: registry.gitlab.com/robigalia/runner:latest
+    cache:
+        key: $CACHE_KEY
+        paths:
+            - /root/.cargo
+    script: 
+        - "xargo build --target i686-sel4-robigalia --no-default-features"
+        - "xargo build --target arm-sel4-robigalia --no-default-features"
+        - "cargo build --no-default-features"
+        - "cargo test"
+        - "cargo test --release"
+#        - "kcov --exclude-pattern=/.cargo,/usr/lib --verify target/cov/ target/debug/roundtrip-*[^\\.d]"
+#        - "bash <(curl -s https://codecov.io/bash) -t 84321081-ebe2-447a-bdf1-a69eee2bdcf7"
+#        - echo "Percent coverage: $(jq .percent_covered target/cov/rn-*/coverage.json)%"
+
+verify_signatures:
+    image: ubuntu:17.04
+    script:
+        - apt update
+        - apt install --assume-yes python-pexpect gnupg2 git
+        - git clone --depth=1 https://gitlab.com/robigalia/meta
+        - gpg2 --list-keys # to force it to create GNUPGHOME instead of racing in the pipe below
+        - (gpg2 --export --no-default-keyring --keyring ./meta/.gnupg/pubring.kbx || true) | gpg2 --import
+        - gpg2 --list-keys --fingerprint --fingerprint --verbose
+        - ./meta/verify-signatures.py HEAD ./meta/.mailmap