added snapshot-based checkpointing for historical balance and supply proofs

[EmeditWeb]

Close #565

PR Summary

• Introduces snapshot-based checkpointing to record historical total assets, total shares, and per-user share balances for proof queries.

Changes

• Updated lib.rs: added checkpoint storage keys and functions:
  • create_checkpoint, total_shares_at, total_assets_at, snapshot_user_balance, balance_at
• Updated test.rs: added unit test test_checkpoint validating checkpoint creation and user snapshot.

Tests

• Added unit test test_checkpoint in the vault tests suite.
• Run full contract tests with cargo test (requires Rust/cargo in environment).

How to test locally

# if Rust/cargo not installed
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
source $HOME/.cargo/env

# run the vault tests
cd contracts/vault
cargo test

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