refactor(engine): audit and rationalize platform cfg gates (closes #49)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

solitaire_engine/src/platform/clipboard.rs

@@ -0,0 +1,72 @@
+use std::sync::Arc;
+
+use bevy::prelude::Resource;
+use thiserror::Error;
+
+/// Abstracts platform-specific clipboard access for gameplay UI systems.
+pub trait ClipboardBackend: Send + Sync + 'static {
+    /// Write plain text to the active OS clipboard.
+    fn set_text(&self, text: &str) -> Result<(), ClipboardError>;
+}
+
+/// Bevy resource that exposes the active clipboard backend.
+#[derive(Resource, Clone)]
+pub struct ClipboardBackendResource(pub Arc<dyn ClipboardBackend>);
+
+/// Errors surfaced by platform clipboard backends.
+#[derive(Debug, Error)]
+pub enum ClipboardError {
+    #[cfg(all(not(target_os = "android"), not(target_arch = "wasm32")))]
+    #[error(transparent)]
+    Native(#[from] arboard::Error),
+    #[cfg(target_os = "android")]
+    #[error("android clipboard failed: {0}")]
+    Android(String),
+    #[cfg(all(target_arch = "wasm32", not(target_os = "android")))]
+    #[error("clipboard backend unavailable on wasm32")]
+    Unsupported,
+}
+
+/// Construct the default clipboard backend for the current platform.
+pub fn default_clipboard_backend() -> Result<Arc<dyn ClipboardBackend>, ClipboardError> {
+    #[cfg(all(not(target_os = "android"), not(target_arch = "wasm32")))]
+    {
+        Ok(Arc::new(NativeClipboardBackend))
+    }
+
+    #[cfg(target_os = "android")]
+    {
+        Ok(Arc::new(AndroidClipboardBackend))
+    }
+
+    #[cfg(all(target_arch = "wasm32", not(target_os = "android")))]
+    {
+        Err(ClipboardError::Unsupported)
+    }
+}
+
+/// `arboard`-backed clipboard bridge for desktop targets.
+#[cfg(all(not(target_os = "android"), not(target_arch = "wasm32")))]
+#[derive(Debug, Default, Clone, Copy)]
+pub struct NativeClipboardBackend;
+
+#[cfg(all(not(target_os = "android"), not(target_arch = "wasm32")))]
+impl ClipboardBackend for NativeClipboardBackend {
+    fn set_text(&self, text: &str) -> Result<(), ClipboardError> {
+        let mut clipboard = arboard::Clipboard::new()?;
+        clipboard.set_text(text.to_string())?;
+        Ok(())
+    }
+}
+
+/// JNI-backed clipboard bridge for Android targets.
+#[cfg(target_os = "android")]
+#[derive(Debug, Default, Clone, Copy)]
+pub struct AndroidClipboardBackend;
+
+#[cfg(target_os = "android")]
+impl ClipboardBackend for AndroidClipboardBackend {
+    fn set_text(&self, text: &str) -> Result<(), ClipboardError> {
+        crate::android_clipboard::set_text(text).map_err(ClipboardError::Android)
+    }
+}

solitaire_engine/src/platform/mod.rs

@@ -1,12 +1,28 @@
 //! Platform abstraction layer.
 //!
-//! Traits defined here are implemented per target:
-//! - native builds use filesystem-backed storage
-//! - browser builds use `localStorage`
+//! Target-specific implementations live here so gameplay and rendering systems
+//! can depend on stable engine-facing abstractions instead of sprinkling
+//! `#[cfg(...)]` branches through UI code.
 
+pub mod clipboard;
 pub mod storage;
 pub mod time;
 
+#[cfg(target_os = "android")]
+/// `false` on touch-first Android builds, where UI buttons replace keyboard chips.
+pub const SHOW_KEYBOARD_ACCELERATORS: bool = false;
+#[cfg(not(target_os = "android"))]
+/// `true` on desktop builds, where keyboard chips should be rendered.
+pub const SHOW_KEYBOARD_ACCELERATORS: bool = true;
+
+#[cfg(target_os = "android")]
+/// `true` when the engine should prefer touch-optimised HUD affordances.
+pub const USE_TOUCH_UI_LAYOUT: bool = true;
+#[cfg(not(target_os = "android"))]
+/// `false` when the engine should prefer desktop HUD affordances.
+pub const USE_TOUCH_UI_LAYOUT: bool = false;
+
+pub use clipboard::{ClipboardBackend, ClipboardBackendResource, default_clipboard_backend};
 #[cfg(not(target_arch = "wasm32"))]
 pub use storage::NativeStorage;
 #[cfg(target_arch = "wasm32")]