Clarify block number source in helpers and consider renaming #176
Description
As pointed out by @jmulq in #175 (comment), event.block.number and RoundsManager.blockNum() are equivalent today (since blockNum() returns block.number), but the helper name getBlockNum() is ambiguous. We should decide which source we want to rely on (event data vs. contract call) for clarity and efficiency, and rename the helper accordingly (e.g., getRoundsManagerBlockNum).
Goal: make the source of truth explicit, improve readability, and avoid unnecessary contract calls or confusion if the implementation diverges in the future.