add eigen package (#9)

Header-only Eigen 3.4.0 C++ linear algebra library, downloaded from
GitLab release. Exposes INCLUDE_DIR for scons integration.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

eigen/build.sh

@@ -0,0 +1,34 @@
+#!/usr/bin/env bash
+set -e
+
+DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null && pwd)"
+cd "$DIR"
+
+VERSION="3.4.0"
+INSTALL_DIR="$DIR/eigen/install"
+
+# Idempotent: skip if already present
+if [ -d "$INSTALL_DIR/eigen3/Eigen" ]; then
+  echo "eigen already present, skipping download."
+  exit 0
+fi
+
+TARBALL="eigen-${VERSION}.tar.gz"
+URL="https://gitlab.com/libeigen/eigen/-/archive/${VERSION}/${TARBALL}"
+
+echo "Downloading Eigen ${VERSION} ..."
+curl -fSL -o "$TARBALL" "$URL"
+
+echo "Extracting headers ..."
+rm -rf "$INSTALL_DIR"
+mkdir -p "$INSTALL_DIR/eigen3"
+
+# Extract only the Eigen/ and unsupported/Eigen/ header directories
+tar --strip-components=1 -xzf "$TARBALL" -C "$INSTALL_DIR/eigen3" \
+  "eigen-${VERSION}/Eigen" \
+  "eigen-${VERSION}/unsupported"
+
+rm -f "$TARBALL"
+
+echo "Installed eigen to $INSTALL_DIR"
+du -sh "$INSTALL_DIR"