fix: security and panic fixes (#127, #137)

[0xMuang]

Summary

• [Security] Weak scrypt KDF parameters for keystore encryption #127: Use EIP-2335 standard scrypt N=262144 instead of weak N=8192 for stronger key derivation security
• Potential panic on subscription_manager.unwrap() in node startup #137: Replace subscription_manager.clone().unwrap() with pattern matching to prevent potential panic

Changes

kdf.rs (Issue #127)

Changed scrypt parameter from N=8192 (2^13) to N=262144 (2^18) to follow EIP-2335 standard. This provides stronger security for key derivation.

node.rs (Issue #137)

Changed the RPC server initialization from:

if let (Some(ref storage), Some(ref debug_executor)) = (&rpc_storage, &rpc_debug_executor) {
    // ...
    subscription_manager.clone().unwrap(),
}

to:

if let (Some(ref storage), Some(ref debug_executor), Some(ref sub_mgr)) =
    (&rpc_storage, &rpc_debug_executor, &subscription_manager)
{
    // ...
    sub_mgr.clone(),
}

This prevents a potential panic if subscription_manager is None.

Test Plan

• cargo check passes
• cargo test -p cipherbft-crypto --lib passes (all 102 tests)

Closes #127
Closes #137