dd contributor guide for deterministic quote tests

[nayt9]

Adds 4 new quote monotonicity tests and comprehensive contributor documentation for CI, storage invariants, and testing best practices.

Changes
Tests (
buy_quote_monotonicity.rs
)
Added 4 tests verifying quote stability under the fixed price model:

test_buy_quote_monotonic_with_zero_creator_fee - 50/50 fee split edge case
test_buy_quote_monotonic_with_zero_protocol_fee - 100% creator fee edge case
test_buy_quote_stable_across_50_sequential_purchases - High-volume stability test
test_buy_quote_multiple_creators_independent_monotonicity - Multi-creator independence
All tests verify that quotes remain constant after purchases (fixed price model).

Documentation
ci-contract-checks.md

• CI pipeline guide, local verification, troubleshooting
  storage-key-invariants.md
• Storage model, key invariants, common pitfalls
  deterministic-quote-tests.md
• Quote testing guide with patterns and examples
  Updated CONTRIBUTING.md - Added links to new documentation
  Key Fixes
  Fee configs now sum to 10,000 bps (100%) with protocol ≤ 5,000 bps (50%)
  Tests use total_amount for payments, not just price
  Tests reflect global price model (shared across creators)
  Test Results
  test result: ok. 14 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
  Verification
  cargo test --test buy_quote_monotonicity
  cargo fmt --all -- --check
  cargo clippy --workspace --all-targets

closes #149
closes #151
closes #155
closes #176

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