chore(vendor): vendor DuckDB v1.1.3 + CMake target DuckDB::DuckDB

cpp/vendor/duckdb/ con:
- include/duckdb.h         (C API, versionado)
- download_duckdb.sh       (descarga libs precompiladas para linux/windows)
- .gitignore               (libduckdb.so, duckdb.dll, duckdb.lib, libduckdb_static.a)
- README.md

cpp/CMakeLists.txt anade target INTERFACE 'duckdb_vendored' (alias
DuckDB::DuckDB) que las apps enlazan con target_link_libraries.
duckdb_copy_runtime(<target>) copia la lib runtime al lado del exe en
build time. Primer consumidor: graph_explorer (issue 0010).

cpp/vendor/duckdb/download_duckdb.sh

@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+# Descarga las libs precompiladas de DuckDB para linux y windows en
+# cpp/vendor/duckdb/{linux,windows}/. El header duckdb.h vive en include/
+# y SI esta versionado.
+#
+# Uso:
+#   ./download_duckdb.sh           # ambas plataformas
+#   ./download_duckdb.sh linux     # solo linux
+#   ./download_duckdb.sh windows   # solo windows
+
+set -euo pipefail
+
+VERSION="v1.1.3"
+HERE="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+TMP="$(mktemp -d)"
+trap "rm -rf $TMP" EXIT
+
+target="${1:-all}"
+
+dl_linux() {
+    echo "[duckdb] downloading linux libs..."
+    curl -sL -o "$TMP/lin.zip" \
+        "https://github.com/duckdb/duckdb/releases/download/${VERSION}/libduckdb-linux-amd64.zip"
+    mkdir -p "$HERE/linux"
+    unzip -o "$TMP/lin.zip" -d "$TMP/lin/" > /dev/null
+    cp "$TMP/lin/libduckdb.so" "$HERE/linux/"
+    echo "[duckdb] -> $HERE/linux/libduckdb.so ($(du -h "$HERE/linux/libduckdb.so" | cut -f1))"
+}
+
+dl_windows() {
+    echo "[duckdb] downloading windows libs..."
+    curl -sL -o "$TMP/win.zip" \
+        "https://github.com/duckdb/duckdb/releases/download/${VERSION}/libduckdb-windows-amd64.zip"
+    mkdir -p "$HERE/windows"
+    unzip -o "$TMP/win.zip" -d "$TMP/win/" > /dev/null
+    cp "$TMP/win/duckdb.dll" "$HERE/windows/"
+    cp "$TMP/win/duckdb.lib" "$HERE/windows/"
+    echo "[duckdb] -> $HERE/windows/duckdb.dll ($(du -h "$HERE/windows/duckdb.dll" | cut -f1))"
+}
+
+case "$target" in
+    linux)   dl_linux ;;
+    windows) dl_windows ;;
+    all)     dl_linux; dl_windows ;;
+    *)       echo "Uso: $0 [linux|windows|all]"; exit 1 ;;
+esac