ci: require protocol version ratchet

[maxw06]

Closes #1480.

Summary

• Add a protocol-version release ratchet that fails CI when quicklendx-contracts/Cargo.toml [package].version changes without increasing PROTOCOL_VERSION.
• Add self-tests for unchanged package versions, missing bumps, valid bumps, and protocol-version decreases.
• Document the ratchet in the README, contract version compatibility guide, and upgrade guide.
• Keep existing non-contract/docs-only PR behavior by applying the existing contract-change filter to the unconditional Cargo test and coverage steps.

Validation

bash scripts/check-protocol-version-bump.test.sh
git diff --check origin/main..fork/codex/version-bump-gate-1480

[Baskarayelu]

thanks for the contribution! our workflow only merges a PR when its author is assigned to the linked issue — please get assigned to an open issue first, then raise the PR. closing as invalid for now 🙏