chore: import upstream snapshot with attribution

scripts/check_warning_budget.sh

@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+repo_root=$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)
+baseline_file="$repo_root/scripts/warning_budget.txt"
+
+usage() {
+  cat <<'USAGE'
+Usage:
+  scripts/check_warning_budget.sh            # fail if warnings exceed baseline
+  scripts/check_warning_budget.sh --update   # update baseline to current warning count
+
+Notes:
+  - Counts Rust compiler lines that begin with "warning:" from `cargo check -q`
+  - Baseline is stored in scripts/warning_budget.txt
+USAGE
+}
+
+if [[ "${1:-}" == "-h" || "${1:-}" == "--help" ]]; then
+  usage
+  exit 0
+fi
+
+if [[ ! -f "$baseline_file" ]]; then
+  echo "error: missing baseline file: $baseline_file" >&2
+  exit 1
+fi
+
+current=$(cd "$repo_root" && CARGO_TERM_COLOR=never cargo check -q 2>&1 | rg -c '^warning:' || printf '0\n')
+baseline=$(tr -d '[:space:]' < "$baseline_file")
+
+if [[ "${1:-}" == "--update" ]]; then
+  printf '%s\n' "$current" > "$baseline_file"
+  echo "Updated warning baseline: $baseline"
+  echo "New warning baseline: $current"
+  exit 0
+fi
+
+if ! [[ "$baseline" =~ ^[0-9]+$ ]]; then
+  echo "error: invalid warning baseline in $baseline_file: '$baseline'" >&2
+  exit 1
+fi
+
+if (( current > baseline )); then
+  echo "Warning budget exceeded: current=$current baseline=$baseline" >&2
+  echo "Run scripts/check_warning_budget.sh --update only after intentional cleanup." >&2
+  exit 1
+fi
+
+if (( current < baseline )); then
+  echo "Warning budget improved: current=$current baseline=$baseline"
+  echo "Consider running: scripts/check_warning_budget.sh --update"
+else
+  echo "Warning budget OK: current=$current baseline=$baseline"
+fi