refactor(android): forbid unsafe workspace-wide, quarantine JNI in app (#91)

Addresses #91: the #90 posture (`deny(unsafe_code)` + three scattered
`#![allow(unsafe_code)]` across solitaire_data and solitaire_engine) punched
unsafe holes into otherwise-pure logic crates. Replace it by *reducing* the
unsafe rather than relocating it, then forbidding it everywhere it can be
forbidden.

Changes:
- solitaire_data: new safe `android_jni` bridge owning the cached `JavaVM`
  and activity `GlobalRef`; exposes `with_env` / `with_activity_env` so
  keystore/clipboard/safe-area never touch a raw handle.
- keystore: drop the `JavaVM::from_raw` init path (now in the app) and
  replace the three `unsafe { JByteArray::from_raw(x.into_raw()) }` casts
  with the safe `JByteArray::from(JObject)` conversion jni 0.21 provides.
- engine clipboard + safe_area: route through the bridge; remove their
  `#![allow(unsafe_code)]` and all `from_raw` calls.
- solitaire_app: becomes the single owner of FFI unsafe. `android_main`
  reconstructs the raw `JavaVM` / activity once (it must, as the cdylib that
  exports `#[unsafe(no_mangle)]`) and registers the safe wrappers. It opts to
  its own `deny`-level lints with two scoped `#[allow(unsafe_code)]`.
- workspace: `unsafe_code` is now `forbid`. Every crate except the app entry
  point is fully unsafe-free.

Net: 7 unsafe sites across three crates collapse to 3 at the OS boundary in
one crate. Verified with host `clippy --workspace -- -D warnings` and an
`aarch64-linux-android` clippy build of solitaire_app (transitively engine +
data); also fixed two latent android-only `collapsible_if` warnings surfaced
in the keystore by the cross-target check.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

solitaire_app/Cargo.toml

@@ -22,6 +22,13 @@ bevy             = { workspace = true }
 solitaire_engine = { workspace = true }
 solitaire_data   = { workspace = true }
 
+# Android-only: the entry point reconstructs the raw `JavaVM` / activity
+# handles and registers the safe `solitaire_data::android_jni` bridge. This
+# is the one crate in the workspace that performs `unsafe` FFI, so it is also
+# the only one that depends on `jni` directly at the app layer.
+[target.'cfg(target_os = "android")'.dependencies]
+jni = { workspace = true }
+
 # Desktop-only deps. `keyring`'s default-store init only matters on
 # platforms with a real keychain backend (Linux Secret Service,
 # macOS Keychain, Windows Credential Store), and its transitive
@@ -100,5 +107,17 @@ icon = "@mipmap/ic_launcher"
 # enabling auto-rotate.
 orientation = "portrait"
 
-[lints]
-workspace = true
+# `solitaire_app` is the one crate that cannot inherit the workspace
+# `forbid(unsafe_code)`: as the Android cdylib it must export the
+# `#[unsafe(no_mangle)]` entry point and reconstruct the raw JNI handles
+# there (a `no_mangle` symbol cannot live in a dependency rlib). It mirrors
+# the workspace lints but at `deny`, so the two `#[allow(unsafe_code)]`
+# scopes in the Android entry point are the only unsafe in the whole tree.
+[lints.rust]
+unsafe_code = "deny"
+single_use_lifetimes = "warn"
+trivial_casts = "warn"
+unused_lifetimes = "warn"
+unused_qualifications = "warn"
+variant_size_differences = "warn"
+unexpected_cfgs = "warn"

solitaire_app/src/lib.rs

@@ -363,16 +363,57 @@ fn set_window_icon(
 /// works on a function named `main`; our shared entry point is `run`, so
 /// we emit the equivalent expansion manually.
 #[cfg(target_os = "android")]
+#[allow(unsafe_code)]
 #[unsafe(no_mangle)]
 fn android_main(android_app: bevy::android::android_activity::AndroidApp) {
-    let vm_ptr = android_app.vm_as_ptr().cast();
-    if let Err(e) = solitaire_data::init_android_jvm(vm_ptr) {
-        eprintln!("warn: could not initialise Android Keystore JNI ({e})");
+    if let Err(e) = init_android_jni(&android_app) {
+        eprintln!("warn: could not initialise Android JNI bridge ({e})");
     }
     let _ = bevy::android::ANDROID_APP.set(android_app);
     run();
 }
 
+/// Reconstructs the raw `JavaVM` / `NativeActivity` handles handed over by the
+/// Android runtime and registers safe wrappers with `solitaire_data`.
+///
+/// This is the *only* place in the workspace that performs `unsafe` FFI handle
+/// reconstruction. Every other crate consumes the safe
+/// [`solitaire_data::android_jni`] bridge and stays `forbid(unsafe_code)`;
+/// `solitaire_app` opts down to `deny` with a narrowly scoped allow on this
+/// function and the `#[unsafe(no_mangle)]` entry point above.
+#[cfg(target_os = "android")]
+#[allow(unsafe_code)]
+fn init_android_jni(
+    android_app: &bevy::android::android_activity::AndroidApp,
+) -> Result<(), String> {
+    use jni::JavaVM;
+    use jni::objects::JObject;
+
+    let vm_ptr = android_app.vm_as_ptr();
+    if vm_ptr.is_null() {
+        return Err("JavaVM pointer is null".into());
+    }
+    // SAFETY: `vm_as_ptr()` returns the process-wide JavaVM* established by the
+    // Android runtime; it is valid for the lifetime of the process.
+    let vm = unsafe { JavaVM::from_raw(vm_ptr.cast()) }.map_err(|e| format!("JavaVM: {e}"))?;
+
+    let env = vm
+        .attach_current_thread_permanently()
+        .map_err(|e| format!("attach_current_thread: {e}"))?;
+
+    // SAFETY: `activity_as_ptr()` returns the NativeActivity jobject pointer,
+    // valid for the lifetime of the process. Promote it to a global reference
+    // so the safe bridge can hand it to any thread.
+    let activity = unsafe { JObject::from_raw(android_app.activity_as_ptr().cast()) };
+    let activity_ref = env
+        .new_global_ref(&activity)
+        .map_err(|e| format!("activity global ref: {e}"))?;
+
+    solitaire_data::android_jni::set_jvm(vm);
+    solitaire_data::android_jni::set_activity(activity_ref);
+    Ok(())
+}
+
 /// Wraps the default panic hook with one that also appends a crash log
 /// to `<data_dir>/crash.log` (next to `settings.json`). The default hook
 /// still runs afterwards, so stderr output and debugger integration are