mempalace-native: default --db path ignores the XDG config directory

What happens

mempalace-native stats, bench and search default --db to ~/.mempalace/palace/sqlite_exact.sqlite3 (crates/mempalace-cli/src/main.rs; resolve_path expands ~ via USERPROFILE/HOME). Since #148, a fresh install keeps its config and palace under the XDG config directory ($MEMPALACE_CONFIG_DIR, a legacy ~/.mempalace holding a real install, $XDG_CONFIG_HOME/mempalace, then ~/.config/mempalace). On such an install the default path does not exist, so the CLI fails unless --db is passed, while the Python tools find the palace.

Expected

The default resolves the same way mempalace.config._default_config_dir() does (and honours a configured palace_path if practical), or the CLI requires --db and says so.

Raised by Copilot on #2519 (review of the v3.10.0 promotion) and verified against develop @ ce54d5d. Not a release blocker: --db works.