feat: test helpers, deploy/consumer docs, sell quote rounding tests

[Abidoyesimze]

Closes #147 Closes #152 Closes #153 Closes #156

Summary

• Add reusable contract test setup helper module #147 — Reusable creator-keys integration test setup in tests/contract_test_env/; refactors buy_key, sell_key, and quote_zero_amount to use shared helpers; documents usage in CONTRIBUTING.md.
• Add deploy artifact storage and naming guidance #152 — Deploy artifact naming, safe storage, metadata, and retention: docs/deploy-artifacts.md; cross-links from docs/stellar-testnet-deployment.md, README, and CONTRIBUTING.
• Add architecture note for client/server dependency on contract outputs #153 — Client/server expectations for contract reads, events, and PROTOCOL_STATE_VERSION: docs/contract-consumer-boundaries.md; linked from README/CONTRIBUTING.
• Add sell quote rounding behavior tests #156 — Deterministic sell quote rounding tests (fee floor, small prices, 50/50, 100% creator, max protocol bps) with snapshots as applicable.

How to verify

cargo fmt --all -- --check
cargo clippy --workspace --all-targets -- -D warnings
cargo test --workspace

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