feat: add cross-contract upgrade compatibility checks

[afeezorobsco-cyber]

##close #96

Summary

Adds explicit cross-contract interface/version compatibility checks to prevent unilateral contract upgrades from silently breaking the prediction market, referral, leaderboard, and token call chain.

What changed

• Added contract/interface version metadata and read access where required.
• Added compatibility validation for cross-contract calls.
• Added typed failures for incompatible or missing interface versions.
• Preserved existing business-function ABI signatures.
• Added tests for compatible, incompatible, and uninitialized version states.
• Documented the upgrade coordination/versioning model and deployment implications.

Protected dependencies

• prediction_market → referral_registry
• prediction_market → leaderboard
• referral_registry → leaderboard
• leaderboard → pulse_token

Validation

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

Upgrade safety

Contract upgrades must preserve the declared interface compatibility expected by their callers. An incompatible target is rejected before the dependent cross-contract operation proceeds.

Closes #[ISSUE_NUMBER]