refactor(data,core): consolidate APP_DIR_NAME and add #[must_use] on pure fns

- Hoist APP_DIR_NAME = "ferrous_solitaire" to solitaire_data crate root
  as pub(crate); remove 5 duplicate local definitions across achievements,
  progress, settings, storage, replay modules (L-9)
- Add #[must_use] to can_place_on_foundation, can_place_on_tableau, and
  is_valid_tableau_sequence in solitaire_core::rules so callers that
  accidentally discard the result get a compile-time warning (L-6)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_data/src/lib.rs

@@ -168,3 +168,6 @@ pub use matomo_client::MatomoClient;
 
 pub mod platform;
 pub use platform::data_dir;
+
+/// Application data subdirectory name, shared by all persistence modules.
+pub(crate) const APP_DIR_NAME: &str = "ferrous_solitaire";