chore: add release script and make target

Usage: make release VERSION=0.8.0
Bumps Cargo.toml + Cargo.lock, commits, tags, pushes — triggers
the existing GitHub Actions release workflow.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/release.sh

@@ -0,0 +1,43 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if [ $# -ne 1 ]; then
+  echo "Usage: $0 <version>  (e.g. 0.7.0)" >&2
+  exit 1
+fi
+
+VERSION="$1"
+TAG="v$VERSION"
+
+# Sanity checks
+if ! git diff --quiet || ! git diff --cached --quiet; then
+  echo "ERROR: working tree is dirty — commit or stash first" >&2
+  exit 1
+fi
+
+if [ "$(git branch --show-current)" != "main" ]; then
+  echo "ERROR: must be on main branch" >&2
+  exit 1
+fi
+
+if git tag -l "$TAG" | grep -q .; then
+  echo "ERROR: tag $TAG already exists" >&2
+  exit 1
+fi
+
+CURRENT=$(grep '^version = ' Cargo.toml | head -1 | sed 's/version = "\(.*\)"/\1/')
+echo "Bumping $CURRENT -> $VERSION"
+
+# Bump version
+sed -i.bak "s/^version = \"$CURRENT\"/version = \"$VERSION\"/" Cargo.toml
+rm -f Cargo.toml.bak
+cargo update --workspace
+
+# Commit, tag, push
+git add Cargo.toml Cargo.lock
+git commit -m "chore: bump version to $VERSION"
+git tag "$TAG"
+git push origin main --tags
+
+echo
+echo "Released $TAG — GitHub Actions will build, publish to crates.io, and create the release."