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/achievements.rs

@@ -10,12 +10,11 @@ use std::path::{Path, PathBuf};
 
 pub use solitaire_sync::AchievementRecord;
 
-const APP_DIR_NAME: &str = "ferrous_solitaire";
 const FILE_NAME: &str = "achievements.json";
 
 /// Platform-specific default path for `achievements.json`.
 pub fn achievements_file_path() -> Option<PathBuf> {
-    crate::data_dir().map(|d| d.join(APP_DIR_NAME).join(FILE_NAME))
+    crate::data_dir().map(|d| d.join(crate::APP_DIR_NAME).join(FILE_NAME))
 }
 
 /// Load achievements from an explicit path. Returns `Vec::new()` if the file