feat(assets): implement virtual AssetManager with non-destructive mod overlays

crates/molotov-core/src/assets/manager.rs

@@ -0,0 +1,71 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+use super::asset::AssetFile;
+use super::dir::AssetDir;
+use std::collections::HashMap;
+
+pub struct AssetManager {
+    base_assets: HashMap<String, Vec<u8>>,
+    mod_overrides: HashMap<String, Vec<u8>>,
+}
+
+impl Default for AssetManager {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl AssetManager {
+    pub fn new() -> Self {
+        Self {
+            base_assets: HashMap::new(),
+            mod_overrides: HashMap::new(),
+        }
+    }
+
+    pub fn add_asset(&mut self, path: &str, data: Vec<u8>) {
+        self.base_assets.insert(path.to_string(), data);
+    }
+
+    pub fn add_mod_override(&mut self, path: &str, data: Vec<u8>) {
+        self.mod_overrides.insert(path.to_string(), data);
+    }
+
+    pub fn open(&self, filename: &str) -> Option<AssetFile> {
+        // 1. Check user mod overrides first (non-destructive mod engine)
+        if let Some(data) = self.mod_overrides.get(filename) {
+            tracing::info!("AssetManager: redirected '{}' to active user mod", filename);
+            return Some(AssetFile::new(data.clone()));
+        }
+
+        // 2. Fallback to base asset
+        if let Some(data) = self.base_assets.get(filename) {
+            return Some(AssetFile::new(data.clone()));
+        }
+
+        tracing::warn!("AssetManager: asset '{}' not found", filename);
+        None
+    }
+
+    pub fn open_dir(&self, dir_name: &str) -> AssetDir {
+        let prefix = if dir_name.is_empty() || dir_name == "." {
+            "".to_string()
+        } else if !dir_name.ends_with('/') {
+            format!("{}/", dir_name)
+        } else {
+            dir_name.to_string()
+        };
+
+        let mut files = Vec::new();
+        for key in self.base_assets.keys().chain(self.mod_overrides.keys()) {
+            if key.starts_with(&prefix) {
+                let rest = &key[prefix.len()..];
+                if !rest.contains('/') {
+                    files.push(rest.to_string());
+                }
+            }
+        }
+        files.sort();
+        files.dedup();
+        AssetDir::new(files)
+    }
+}

crates/molotov-core/src/assets/mod.rs

@@ -1,4 +1,5 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
 pub mod asset;
 pub mod dir;
+pub mod manager;
 pub mod types;

crates/molotov-core/tests/asset_manager_virtual_test.rs

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+use molotov_core::assets::manager::AssetManager;
+
+#[test]
+fn test_asset_manager_open_with_mod_override() {
+    let mut mgr = AssetManager::new();
+    mgr.add_asset("content/sounds/ouch.ogg", b"BASE_SOUND".to_vec());
+    mgr.add_mod_override("content/sounds/ouch.ogg", b"CUSTOM_MOD_SOUND".to_vec());
+
+    let mut asset = mgr.open("content/sounds/ouch.ogg").expect("Should open asset");
+    let mut content = vec![0u8; asset.length() as usize];
+    asset.read(&mut content);
+
+    assert_eq!(&content, b"CUSTOM_MOD_SOUND", "Must redirect to user mod bytes");
+}