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_todo.sh

@@ -0,0 +1,82 @@
+#!/usr/bin/env bash
+#
+# Copyright 2025 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 -euo pipefail
+
+# This allows us to leave XODO comments in this file and have them still be
+# picked up by this script without having the script itself trigger false
+# positives. The alternative would be to exclude this script entirely, which
+# would mean that we couldn't use XODO comments in this script.
+KEYWORD=$(echo XODO | sed -e 's/X/T/')
+
+# Make sure `rg` is installed (if this fails, `set -e` above will cause the
+# script to exit).
+rg --version >/dev/null
+
+# -H: Print filename (default for multiple files/recursive)
+# -n: Print line number
+# -w: Match whole words
+# Match TODO, TODO-check-disable, and TODO-check-enable
+PATTERN="$KEYWORD|$KEYWORD-check-disable|$KEYWORD-check-enable"
+output=$(rg -H -n -w "$PATTERN" "$@" || true)
+
+commit_output=$(git log -1 --pretty=%B 2>/dev/null | rg -n -w "$PATTERN" || true)
+if [ -n "$commit_output" ]; then
+  commit_output=$(echo "$commit_output" | sed "s/^/COMMIT_MESSAGE:/")
+  output="$commit_output${output:+$'\n'$output}"
+fi
+
+if [ -z "$output" ]; then
+  exit 0
+fi
+
+# Track the disabled state for each file. Since we process lines in order for
+# each file, we can maintain state. However, rg output might interleave files if
+# not sorted, but usually it's grouped. To be safe, we sort the output by
+# filename and line number.
+sorted_output=$(echo "$output" | sort -t: -k1,1 -k2,2n)
+
+exit_code=0
+current_file=""
+disabled=0
+
+while IFS= read -r line; do
+  if [[ "$line" =~ ^(.*):([0-9]+):(.*)$ ]]; then
+    file="${BASH_REMATCH[1]}"
+    content="${BASH_REMATCH[3]}"
+  else
+    echo "Error: could not parse rg output line: $line" >&2
+    exit 1
+  fi
+
+  if [ "$file" != "$current_file" ]; then
+    current_file="$file"
+    disabled=0
+  fi
+
+  if [[ "$content" == *"$KEYWORD-check-disable"* ]]; then
+    disabled=1
+  elif [[ "$content" == *"$KEYWORD-check-enable"* ]]; then
+    disabled=0
+  elif [[ "$content" == *"$KEYWORD"* ]]; then
+    if [ "$disabled" -eq 0 ]; then
+      if [ "$exit_code" -eq 0 ]; then
+        echo "Found $KEYWORD markers in the codebase." >&2
+        echo "$KEYWORD is used for tasks that should be done before merging a PR; if you want to leave a message in the codebase, use FIXME." >&2
+        echo "If you need to allow a $KEYWORD, wrap it in $KEYWORD-check-disable and $KEYWORD-check-enable." >&2
+        echo "" >&2
+        exit_code=1
+      fi
+      echo "$line" >&2
+    fi
+  fi
+done <<< "$sorted_output"
+
+exit $exit_code