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/CMakeLists.txt

@@ -107,6 +107,39 @@ if(NOT SQLite3_FOUND AND NOT TARGET sqlite3_vendored)
     add_library(SQLite::SQLite3 ALIAS sqlite3_vendored)
 endif()
 
+# --- DuckDB (precompiled libs — see cpp/vendor/duckdb/README.md) ---
+# Header en vendor/duckdb/include/. Lib dinamica en linux/ o windows/ segun
+# el target. Las libs estan gitignored — ejecutar
+# `cpp/vendor/duckdb/download_duckdb.sh` la primera vez.
+if(NOT TARGET duckdb_vendored)
+    set(DUCKDB_DIR ${CMAKE_CURRENT_SOURCE_DIR}/vendor/duckdb)
+    if(WIN32)
+        set(DUCKDB_LIB     ${DUCKDB_DIR}/windows/duckdb.lib)
+        set(DUCKDB_RUNTIME ${DUCKDB_DIR}/windows/duckdb.dll)
+    else()
+        set(DUCKDB_LIB     ${DUCKDB_DIR}/linux/libduckdb.so)
+        set(DUCKDB_RUNTIME ${DUCKDB_DIR}/linux/libduckdb.so)
+    endif()
+    if(NOT EXISTS ${DUCKDB_LIB})
+        message(WARNING "[duckdb] ${DUCKDB_LIB} no existe — corre cpp/vendor/duckdb/download_duckdb.sh")
+    endif()
+    add_library(duckdb_vendored INTERFACE)
+    target_include_directories(duckdb_vendored INTERFACE ${DUCKDB_DIR}/include)
+    target_link_libraries(duckdb_vendored INTERFACE ${DUCKDB_LIB})
+    add_library(DuckDB::DuckDB ALIAS duckdb_vendored)
+
+    # Helper para que las apps copien la runtime lib al lado del exe.
+    # add_imgui_app puede invocarlo, o cada app lo hace en su CMakeLists.
+    function(duckdb_copy_runtime target)
+        if(EXISTS ${DUCKDB_RUNTIME})
+            add_custom_command(TARGET ${target} POST_BUILD
+                COMMAND ${CMAKE_COMMAND} -E copy_if_different
+                        ${DUCKDB_RUNTIME} $<TARGET_FILE_DIR:${target}>
+                COMMENT "Copying DuckDB runtime next to ${target}")
+        endif()
+    endfunction()
+endif()
+
 # --- Framework ---
 # Incluye tokens.cpp (identidad visual Mantine dark + indigo), icon_font.cpp
 # (Karla/Roboto/... + Tabler), app_settings.cpp (persistencia y ventana de