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/settings.rs

@@ -11,7 +11,6 @@ use std::path::{Path, PathBuf};
 use serde::{Deserialize, Serialize};
 use solitaire_core::game_state::{DifficultyLevel, DrawMode};
 
-const APP_DIR_NAME: &str = "ferrous_solitaire";
 const SETTINGS_FILE_NAME: &str = "settings.json";
 
 /// Animation playback speed for card transitions.
@@ -479,7 +478,7 @@ impl Settings {
 /// Returns the platform-specific path to `settings.json`, or `None` if
 /// the platform's data directory is unavailable.
 pub fn settings_file_path() -> Option<PathBuf> {
-    crate::data_dir().map(|d| d.join(APP_DIR_NAME).join(SETTINGS_FILE_NAME))
+    crate::data_dir().map(|d| d.join(crate::APP_DIR_NAME).join(SETTINGS_FILE_NAME))
 }
 
 /// Load settings from an explicit path. Returns `Settings::default()` if the