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/cortex-m-rt/ci/script.sh

@@ -0,0 +1,87 @@
+#!/usr/bin/env bash
+
+set -euxo pipefail
+
+main() {
+    cargo check --target "$TARGET"
+
+    cargo check --target "$TARGET" --features device
+
+    # A `critical_section` implementation is always needed.
+    needed_features=cortex-m/critical-section-single-core
+
+    if [ "$TARGET" = x86_64-unknown-linux-gnu ] && [ "$TRAVIS_RUST_VERSION" = stable ]; then
+        ( cd macros && cargo check && cargo test )
+
+        cargo test --features "device,${needed_features}" --test compiletest
+    fi
+
+    local examples=(
+        alignment
+        divergent-default-handler
+        divergent-exception
+        entry-static
+        hard-fault-trampoline
+        main
+        minimal
+        override-exception
+        pre_init
+        qemu
+        state
+        unsafe-default-handler
+        unsafe-entry
+        unsafe-exception
+        unsafe-hard-fault
+    )
+    local fail_examples=(
+        data_overflow
+    )
+    local linkers=(
+        # Link with arm-none-eabi-ld
+        "-C linker=arm-none-eabi-ld"
+        # Link with arm-none-eabi-gcc, requires -nostartfiles
+        "-C linker=arm-none-eabi-gcc -C link-arg=-nostartfiles"
+        # Link with rust-lld (default)
+        ""
+    )
+    if [ "$TARGET" != x86_64-unknown-linux-gnu ]; then
+        # Only test on stable and nightly, not MSRV.
+        if [ "$TRAVIS_RUST_VERSION" = stable ] || [ "$TRAVIS_RUST_VERSION" = nightly ]; then
+            RUSTDOCFLAGS="-Cpanic=abort" cargo test --features "${needed_features}" --doc
+        fi
+
+        for linker in "${linkers[@]}"; do
+            for ex in "${examples[@]}"; do
+                cargo rustc --target "$TARGET" --example "$ex" --features "${needed_features}" -- $linker
+                cargo rustc --target "$TARGET" --example "$ex" --features "${needed_features}" --release -- $linker
+            done
+            for ex in "${fail_examples[@]}"; do
+                cargo rustc --target "$TARGET" --example "$ex" --features "${needed_features}" -- $linker && exit 1
+                cargo rustc --target "$TARGET" --example "$ex" --features "${needed_features}" --release -- $linker && exit 1
+            done
+            cargo rustc --target "$TARGET" --example device --features "device,${needed_features}" -- $linker
+            cargo rustc --target "$TARGET" --example device --features "device,${needed_features}" --release -- $linker
+
+            cargo rustc --target "$TARGET" --example minimal --features "set-sp,${needed_features}" -- $linker
+            cargo rustc --target "$TARGET" --example minimal --features "set-sp,${needed_features}" --release -- $linker
+            cargo rustc --target "$TARGET" --example minimal --features "zero-init-ram,${needed_features}" -- $linker
+            cargo rustc --target "$TARGET" --example minimal --features "zero-init-ram,${needed_features}" --release -- $linker
+            cargo rustc --target "$TARGET" --example minimal --features "set-vtor,${needed_features}" -- $linker
+            cargo rustc --target "$TARGET" --example minimal --features "set-vtor,${needed_features}" --release -- $linker
+        done
+    fi
+
+    case $TARGET in
+        thumbv6m-none-eabi|thumbv7m-none-eabi)
+            for linker in "${linkers[@]}"; do
+                env RUSTFLAGS="$linker -C link-arg=-Tlink.x" cargo run \
+                    --target "$TARGET" --features "${needed_features}" --example qemu | grep "x = 42"
+                env RUSTFLAGS="$linker -C link-arg=-Tlink.x" cargo run \
+                    --target "$TARGET" --features "${needed_features}" --example qemu --release | grep "x = 42"
+            done
+
+            ;;
+    esac
+}
+
+main