test: cover farming pool boost credit invariants

[jamilahmadzai]

Closes #22

Bounty: GrantFox OSS campaign for issue #22

Summary

• add exact boost arithmetic coverage for full allocation at 3x and 1x multipliers
• assert boost and multiplier event payloads, admin authorization, and non-retroactive checkpointing
• cover monotonic get_credits, boosted-vs-unboosted accrual, stake/lock credit separation, and stake/unstake balance round trip

Validation

• cargo fmt --all -- --check
• cargo build
• cargo test -p farming-pool boost -- --nocapture
• cargo test -p farming-pool -- --nocapture
• cargo test --workspace -- --nocapture
• git diff --check

Security notes

• test-only change; no contract runtime behavior changed
• verifies boost and multiplier updates checkpoint old credit windows instead of retroactively recalculating banked credits