Summary

• Adds a shared creator-keys integration test helper for sequential buys. The helper performs a caller-defined number of buys, starts from a provided payment amount, increments each subsequent payment by a provided step, and returns the resulting creator/holder state snapshot.
• Updates existing buy quote monotonicity tests to use the shared helper and assert the cumulative supply and buyer key balance after each helper-driven sequence.

Issue

• Closes Add helper for building a buy sequence fixture with incrementing amounts #366

Changes

• Added perform_incrementing_buys to creator-keys/tests/contract_test_env/mod.rs.
• Replaced the local buy_n helper in buy_quote_monotonicity.rs with the shared incrementing helper.
• Added assertions for helper-driven cumulative supply, buyer balance, and emitted buy event payment values to confirm the incrementing sequence.

Validation

• Ran cargo fmt.
• Ran git diff --check.
• Ran cargo test -p creator-keys test_buy_quote_price_unchanged_after_five_buys but compilation could not proceed because this Windows environment is missing the MSVC linker link.exe before repo code was compiled.

Notes

• The test failure appears environment-related: Rust MSVC targets require Visual Studio Build Tools with the C++ linker installed.