fix(data): route data_dir() through a per-platform shim so Android persists

dirs::data_dir() returns None on Android, which silently disabled
every persistence path (settings, stats, achievements, replays,
game-state, time-attack sessions, user themes). New
solitaire_data::platform::data_dir() shim falls through to
dirs::data_dir() on desktop and returns the per-app sandbox at
/data/data/com.solitairequest.app/files on Android — no JNI needed,
since the package id is pinned in
[package.metadata.android].

CLAUDE.md §10 already flagged this as a known pitfall; the shim
pays it down at the one chokepoint instead of per feature.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_data/src/platform.rs

@@ -0,0 +1,92 @@
+//! Per-platform resolution of the per-user data directory.
+//!
+//! The rest of `solitaire_data` (settings, stats, achievements,
+//! replays, progress, game state) and the engine's user-themes
+//! discovery all need a base path under which to nest
+//! `solitaire_quest/<file>`. On desktop the right answer is
+//! `dirs::data_dir()` (which resolves to platform-appropriate
+//! locations: `~/.local/share` on Linux, `~/Library/Application
+//! Support` on macOS, `%APPDATA%` on Windows). On Android the
+//! `dirs` crate returns `None`, which would silently disable
+//! every persistence path — settings, stats, replays, the lot.
+//!
+//! [`data_dir`] is a thin shim that returns the right base path
+//! per target. Callers continue to append
+//! `solitaire_quest/<file>` themselves, so the on-disk layout is
+//! identical across platforms (the per-app Android sandbox makes
+//! the extra `solitaire_quest/` segment harmless, and a `tar`
+//! export from one platform deserialises cleanly on another).
+//!
+//! # Why hardcode on Android?
+//!
+//! The "proper" Android answer is JNI: call back into Java to
+//! invoke `Activity.getFilesDir()`. That requires plumbing an
+//! `AndroidApp` context through Bevy's startup hooks and a
+//! per-call JNI bridge — meaningfully more code than the
+//! sandbox-guaranteed `/data/data/<package>/files` path. The
+//! package name `com.solitairequest.app` is fixed at compile
+//! time in `solitaire_app/Cargo.toml`'s
+//! `[package.metadata.android]` block, so a hardcoded path is
+//! safe until that ever changes (at which point this constant
+//! moves with it).
+
+use std::path::PathBuf;
+
+/// Hardcoded per-app private files directory on Android.
+///
+/// Matches `[package.metadata.android]` in `solitaire_app/Cargo.toml`.
+/// The Android sandbox guarantees this path exists, is writable,
+/// and is private to the app — no JNI needed. Update both this
+/// constant and the Cargo metadata together if the package id
+/// ever changes.
+#[cfg(target_os = "android")]
+const ANDROID_APP_FILES_DIR: &str = "/data/data/com.solitairequest.app/files";
+
+/// Returns the per-user data directory for the current target,
+/// or `None` if the platform doesn't expose one (rare; usually
+/// indicates a broken `$HOME` or `$XDG_*` configuration on a
+/// minimal Linux container).
+///
+/// Callers append `solitaire_quest/<file>` themselves. See the
+/// module-level doc comment for the per-platform behaviour and
+/// why Android uses a hardcoded path.
+pub fn data_dir() -> Option<PathBuf> {
+    #[cfg(target_os = "android")]
+    {
+        Some(PathBuf::from(ANDROID_APP_FILES_DIR))
+    }
+    #[cfg(not(target_os = "android"))]
+    {
+        dirs::data_dir()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// On every supported desktop target the OS reports a usable
+    /// data directory. This test only runs on desktop because the
+    /// Android branch returns a fixed string regardless of host
+    /// state, and asserting on a fixed string is a tautology.
+    #[cfg(any(target_os = "linux", target_os = "macos", target_os = "windows"))]
+    #[test]
+    fn data_dir_returns_some_on_desktop_targets() {
+        let dir = data_dir().expect("desktop targets must report a data dir");
+        assert!(
+            dir.is_absolute(),
+            "data_dir() must return an absolute path on desktop, got {dir:?}",
+        );
+    }
+
+    /// On Android the hardcoded path matches the package id pinned
+    /// in `solitaire_app/Cargo.toml`'s `[package.metadata.android]`.
+    /// If a future change rotates that id, this test fails loudly
+    /// so the path constant moves with it.
+    #[cfg(target_os = "android")]
+    #[test]
+    fn data_dir_returns_sandbox_path_on_android() {
+        let dir = data_dir().expect("android must report a data dir");
+        assert_eq!(dir, PathBuf::from("/data/data/com.solitairequest.app/files"));
+    }
+}