add justfile with useful commands

justfile

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+# Set the Cargo.toml version, commit it, and tag the commit with that version.
+# Usage: just tag 1.2.3
+tag ver:
+    #!/usr/bin/env bash
+    set -euo pipefail
+
+    # Validate the version argument.
+    if [[ ! "{{ver}}" =~ ^[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
+        echo "error: version must be in the form X.Y.Z (got '{{ver}}')" >&2
+        exit 1
+    fi
+
+    # Bump the version in Cargo.toml.
+    sed -i -E "s/^version = \".*\"/version = \"{{ver}}\"/" Cargo.toml
+
+    # Regenerate Cargo.lock so it reflects the new version.
+    cargo build
+
+    # Stage, commit, and tag.
+    git add Cargo.toml Cargo.lock
+    git commit -m "version {{ver}} [skip ci]"
+    git tag "{{ver}}"
+
+# Run the Rust unit tests.
+# Usage: just test
+test:
+    cargo test --all
+
+# Format the Rust code.
+# Usage: just fmt
+fmt:
+    cargo fmt --all
+
+# Run the end-to-end Playwright tests.
+# Usage: just e2e
+e2e:
+    cd e2e && npm test