docs: add a compatibility matrix of soroban-sdk versions this workspace has been tested against

[Idaonoli]

Scope

rust-toolchain.toml pins a Rust version and Cargo.toml pins soroban-sdk = "27.0.0", but there's no record of which SDK versions have actually been exercised against this workspace's test suite — useful context for anyone considering a version bump, especially since this session found API-shape differences (event topic structure, crypto verification error handling) that suggest the contracts were partly written against an earlier SDK version than what's currently pinned.

Acceptance criteria

• Add a short compatibility note (README or a new doc) recording the current pinned soroban-sdk version and any known API-shape assumptions in the contracts/tests that would need re-verifying on a future bump