Implement membership and subscription contract

[Xaxxoo]

Summary

• Add a Soroban membership subscription workspace contract with configurable tiers and benefit flags.
• Implement trials, pull-based auto-renewals, graceful payment-failure expiration, cancellation refunds, pause/resume, tier changes with proration, referral rewards, entitlement checks, and analytics.
• Add token-backed unit tests and document the new module in the README.

Tests

• cargo fmt --all --check
• cargo test -p membership-subscription --lib -j 1
• cargo build --all --locked -j 1
• cargo clippy --all --all-targets --all-features -j 1 -- -D warnings

Closes #267