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/zerocopy/ci/check_stale_stderr.sh

@@ -0,0 +1,53 @@
+#!/usr/bin/env bash
+#
+# Copyright 2026 The Fuchsia Authors
+#
+# Licensed under a BSD-style license <LICENSE-BSD>, Apache License, Version 2.0
+# <LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0>, or the MIT
+# license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your option.
+# This file may not be copied, modified, or distributed except according to
+# those terms.
+
+set -eo pipefail
+
+# Directories to search
+DIRS=("tests" "zerocopy-derive/tests")
+
+EXIT_CODE=0
+
+for dir in "${DIRS[@]}"; do
+    if [ ! -d "$dir" ]; then
+        echo "Warning: Directory $dir does not exist, skipping." >&2
+        continue
+    fi
+
+    # Find all .stderr files
+    while IFS= read -r -d '' stderr_file; do
+        # Strip .stderr extension
+        base="${stderr_file%.stderr}"
+
+        # Strip toolchain suffixes
+        base="${base%.msrv}"
+        base="${base%.stable}"
+        base="${base%.nightly}"
+
+        # Construct the corresponding .rs file path
+        rs_file="${stderr_file%.stderr}.rs"
+        rs_file="${base}.rs"
+
+        # Check if the .rs file exists. The `-e` flag checks if file exists:
+        # It returns true for regular files and valid symlinks, and false for
+        # broken symlinks or missing files.
+        if [ ! -e "$rs_file" ]; then
+            echo "Error: Orphaned stderr file found: $stderr_file" >&2
+            echo "       Missing regular file or valid symlink: $rs_file" >&2
+            EXIT_CODE=1
+        fi
+    done < <(find "$dir" -name "*.stderr" -print0)
+done
+
+if [ "$EXIT_CODE" -eq 1 ]; then
+    echo "Found stale .stderr files. Please delete them." >&2
+fi
+
+exit "$EXIT_CODE"