Implement State Persistence via RocksDB Backend

[Steake]

Part of Epic: #69 - RC1: Core Transaction & State Infrastructure

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Goal: Persist blockchain state and account data across node restarts using RocksDB.

Requirements:

• Replace in-memory HashMap backend with RocksDB in bitcell-state module
• Store Account and BondState data serialized for persistence
• Update StateManager to use RocksDB
• Ensure transactions update and reflect real state even after restart

Files:

• crates/bitcell-state/src/storage.rs

Acceptance Criteria:

• State persists correctly across node shutdown/restarts
• No data loss in normal operation

Refer to [todo_now.md](https://github.com/Steake/BitCell/blob/master/todo_now. md) for critical path.

[Steake]

@copilot please work on this issue.

Dependencies completed:

• No dependencies

Please create a PR when ready.

[Steake]

⚠️ This issue references other issues (#69).

It is being treated as a blocked epic until those child issues are closed.
Once the referenced issues are completed, this epic can move to in-progress for implementation.