Add snapshot docs, concurrent streams test, cumulative fee rounding d…

[Mikey-222]

closes #1108
closes #1105
closes #1107
closes #1106

• Add test_snapshots/README.md explaining snapshot purpose, regeneration command (SOROBAN_TEST_SNAPSHOTS=1 cargo test), and PR diff workflow
• Link to it from contracts/stream_contract/README.md
• Add test_concurrent_streams_same_tuple_independent_state: verifies two streams with identical (sender, recipient, token) maintain independent withdrawn_amount and deposited_amount
• Add test_cumulative_fee_rounding_drift: 200 small sequential top-ups, asserts cumulative fee drift stays within tolerance (rounds down, favouring user)
• Note: update_fee_config MAX_FEE_RATE_BPS rejection already tested at test_update_fee_config_rejects_invalid_fee_rate (line 171)

Description

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🔧 Refactoring (no functional changes)
• ⚡ Performance improvement
• 🧪 Test addition or update

Related Issues

Closes #

Changes Made

Testing

Test Coverage

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed

Test Steps

Breaking Changes

Breaking Changes:

Migration Guide:

Screenshots/Demo

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published
• I have checked for breaking changes and documented them if applicable

Additional Notes