fix(settlement): checked arithmetic for credited balances

[Agbasimere]

Closes: #134

Summary

This pull request replaces naive i128 balance updates (using += and -=) with checked arithmetic in both the callora-vault and the newly renamed callora-settlement contract (formerly revenue_pool). This ensures any overflow or underflow results in an immediate panic and transaction revert, preventing silent state corruption.

Key Changes

• Contract Renaming: Renamed contracts/revenue_pool to contracts/settlement and updated the crate name to callora-settlement.
• Checked Arithmetic:
  • Replaced meta.balance += amount in vault/src/lib.rs with checked_add().expect("balance overflow").
  • Replaced meta.balance -= amount in vault/src/lib.rs with checked_sub().expect("balance underflow").
  • Replaced total_amount += amount in settlement/src/lib.rs with checked_add().expect("total amount overflow").
• Boundary Tests: Added explicit overflow and underflow tests to vault/src/test.rs and settlement/src/test.rs to verify panic behavior.
• Documentation:
  • Created SECURITY.md documenting the checked arithmetic policy.
  • Updated README.md, INVARIANTS.md, and .github/workflows/ci.yml for the contract rename.
• Workspace Compliance: Ensured overflow-checks = true is explicitly documented and enforced for both dev and release profiles in root Cargo.toml.

Security Notes

All balance mutations now utilize explicit checked arithmetic (checked_add/checked_sub). In the event of an overflow, the contract will panic with a descriptive message, providing an additional layer of security beyond the default workspace policy.

Test Results

• cargo fmt: Passed.
• cargo clippy: Passed with zero warnings.
• cargo test: Added deposit_overflow_panics and batch_distribute_overflow_panics tests; all related tests passing.

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[Agbasimere]

@greatest0fallt1me Kindly review.