test: cover farming pool boost credit invariants

[jamilahmadzai]

Closes #22

Bounty: GrantFox OSS campaign for issue #22; no fixed public award has been confirmed.

Summary

• Add exact boost arithmetic coverage for full allocation at 3x and 1x multipliers.
• Assert boost and multiplier event payloads, admin authorization, and banked-credit preservation across explicit checkpoints.
• Cover monotonic credit growth, boosted-versus-unboosted accrual, aggregate credits versus the dedicated stake/position views, and stake/unstake balance round trips.
• Refresh against current main, including the active-stake requirement for boosts and the old/new multiplier event payload.

Validation (September 8 refresh)

• cargo fmt --manifest-path soroban/Cargo.toml --all -- --check
• cargo build --manifest-path soroban/Cargo.toml --target wasm32v1-none --release -p farming-pool
• cargo test --manifest-path soroban/Cargo.toml -p farming-pool --lib — 164 passed
• git diff --check

This is a test-only contribution; contract runtime behavior is unchanged. The banked-credit test explicitly checkpoints after a same-ledger multiplier update to adopt the new per-stake snapshot.

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[jamilahmadzai]

Refreshed against current main and aligned the coverage with the active-stake requirement, old/new multiplier event payload, and combined versus per-position credit views. The banked-credit case explicitly adopts the new per-stake snapshot at a checkpoint. No contract runtime changes were added. All 164 farming-pool tests pass locally, and the current-head Test and build WASM workflow and deployment preview both pass. Ready for review.