[FEATURE] Implement a `storage` abstraction layer

[takasaki404]

[FEATURE] Implement a storage abstraction layer

📋 Summary

Provide a platform-agnostic storage interface so that Amico can persist data in environments where the OS file system is not available (e.g. WASM in the browser, in-memory caches, databases, embedded systems).

🐞 Problem

• Currently, Amico uses the local OS file system to read/write all files.
• In non-OS environments (WASM/browser, embedded, serverless, etc.), there is no traditional file system API.
• We have no unified way to plug in alternative storage backends (IndexedDB, LocalStorage, in-memory, SQL/NoSQL DB, etc.).

💡 Proposed Solution

1. Define a storage interface in the amico-core crate:

   /// Abstract key/value storage where `namespace` groups tables or
   /// directories, and each key maps to a byte array.
   pub trait Storage {
       /// Read raw bytes for `key` in `namespace`. Returns None if missing.
       fn get(&self, namespace: &str, key: &str) -> Result<Option<Vec<u8>>, StorageError>;
   
       /// Write `value` bytes for `key` in `namespace`.
       fn put(&mut self, namespace: &str, key: &str, value: Vec<u8>) -> Result<(), StorageError>;
   
       /// Delete `key` in `namespace`.
       fn delete(&mut self, namespace: &str, key: &str) -> Result<(), StorageError>;
   
       /// List all keys in `namespace`.
       fn list_keys(&self, namespace: &str) -> Result<Vec<String>, StorageError>;
   }

2. Provide OS filesystem implementation (fs backend):

   • In dev mode: create/locate .amico/ under the project root.
   • In release mode: use a per-user data dir ($HOME/.local/share/amico/ on Linux, %APPDATA%\Amico\ on Windows, ~/Library/Application Support/Amico/ on macOS).
   • Map each namespace to a subfolder; each key to a file name (e.g. <namespace>/<key>.bin).

3. Provide WASM/browser implementation (browser backend):

   • Feature-gated via cfg(target_arch = "wasm32").

   • Use IndexedDB (preferred) or LocalStorage fallback:

     • Map namespace to object store name; key to object key.
     • Serialize bytes to Uint8Array under the hood.

4. Hook up selection logic

   pub enum StorageBackend {
       Fs(FsStorage),
       Browser(BrowserStorage),
       // future: InMem(InMemStorage), Db(DbStorage), etc.
   }
   
   impl Storage for StorageBackend {
       /* delegate to chosen backend */
   }

5. Future backends (to implement later in separate PRs):

   • In-memory (for testing)
   • SQL/NoSQL database (Postgres, SQLite, DynamoDB, etc.)
   • Cloud object stores (S3, GCS)
   • Embedded flash/microcontroller FS

✅ Acceptance Criteria

• amico-core exposes a Storage trait with get/put/delete/list_keys.

• FsStorage works on Linux, macOS, and Windows (correct dev vs. release directory).

• BrowserStorage compiles under wasm32-unknown-unknown and passes integration tests in browser.

• CI matrix includes tests for both fs and browser backends.

• Documentation added:

  • Storage trait doc comments.
  • Example usage in README or amico-cli demo.

• (Optional) Add a simple in-memory backend for unit testing.