feat: add deterministic storage snapshot diff helper

[Deyanju23]

Closes #505

Summary

Adds a reusable helper for diffing two storage snapshots and producing a deterministic, compact list of changes suitable for external reporting tools and CI automation.

Changes

Snapshot Diff Helper

Added:

• contracts/helpers/src/snapshot_diff.rs

Implemented a helper that:

• compares two storage snapshots
• detects added keys
• detects removed keys
• detects modified values
• ignores unchanged entries
• produces a compact diff representation
• guarantees deterministic ordering of output

Cargo Integration

Updated:

• contracts/helpers/Cargo.toml

Registered the new helper module and any required dependencies following the project's existing conventions.

Tests

Added comprehensive tests covering:

• identical snapshots
• added entries
• removed entries
• modified values
• multiple simultaneous changes
• empty snapshots
• deterministic output ordering
• realistic storage snapshot fixtures

Documentation

Added Rust /// documentation explaining:

• helper purpose
• diff semantics
• output format
• ordering guarantees
• intended usage by reporting tools

Security

• Read-only helper.
• No contract state changes.
• No authentication behavior modified.
• No unwrap() introduced in production code.
• Overflow-safe implementation maintained.

Testing

Executed:

cargo test
cargo clippy --workspace --all-targets -- -D warnings

Notes

• Helper-only addition.
• No behavioral changes to contracts.
• No unrelated refactoring.

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[GBOYEE]

@GBOYEE has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Implement following existing patterns and conventions. Add tests, ensure CI passes, and document any new API surface.

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