Soroban protocol compatibility matrix doc

[nanaf6203-bit]

Problem Statement
The contract is built against soroban-sdk = "22.0.0" and pinned to
nightly Rust. There is no doc that states the minimum-supported
Soroban protocol version or the roadmap for upgrading.

Why it matters
Deployment on a Stellar network that doesn't speak the expected
Soroban protocol can cause subtle failures. Indexers and wallets
consuming our events need a stable contract-emitted version.

Technical Context
Cargo.toml pins soroban-sdk. rust-toolchain.toml pins nightly.

Expected Outcome
docs/protocol-compatibility.md table:
| Soroban SDK | Soroban Protocol | Rust | Testnet? | Mainnet? |

Acceptance Criteria

• Table includes the version currently targeted, plus the next two
  planned upgrades.
• Linked from README.

Files/Modules Likely Affected

• docs/protocol-compatibility.md (new)

Difficulty Easy. Estimated Effort XS.

Labels
area:documentation, area:smart-contract-safety,
priority:low

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Imported from docs/open-issues-draft.md as draft issue #24 (commit 7becd4b). Original draft-order cross-references in this body have been remapped to current GitHub issue numbers at import time.

[GBOYEE]

Hi team — I'd like to take this on as part of the GrantFox campaign. I'll write the documentation and open a PR shortly.