feat: Refactor kvstore and vault to use features and logging

- Remove hardcoded dependencies in kvstore Cargo.toml; use features
  instead. This allows for more flexible compilation for different
  targets (native vs. WASM).
- Improve logging in vault crate using the `log` crate. This makes
  debugging easier and provides more informative output during
  execution.  Native tests use `env_logger`, WASM tests use
  `console_log`.
- Update README to reflect new logging best practices.
- Add cfg attributes to native and wasm modules to improve clarity.
- Update traits.rs to specify Send + Sync behavior expectations.

kvstore/src/traits.rs

@@ -16,6 +16,11 @@ use crate::error::Result;
 /// - contains_key (was exists)
 /// - keys
 /// - clear
+/// Async key-value store interface for both native and WASM backends.
+///
+/// For native (non-wasm32) backends, implementers should be `Send + Sync` to support async usage.
+/// For WASM (wasm32) backends, `Send + Sync` is not required.
+#[async_trait::async_trait]
 pub trait KVStore {
     async fn get(&self, key: &str) -> Result<Option<Vec<u8>>>;
     async fn set(&self, key: &str, value: &[u8]) -> Result<()>;