CI matrix: minimum and current Soroban protocol

[nanaf6203-bit]

Problem Statement
There is one CI job that builds with the pinned testbed. A separate
job that confirms the contract functions under the protocol minimum is
missing.

Why it matters
If the contract uses a host function introduced in protocol X,
deploys to a network below X will silently fail in production.

Acceptance Criteria

• A CI step that asserts compilation
  soroban-sdk = "= MIN_PROTOCOL_SDK" succeeds.
• Documented in docs/protocol-compatibility.md (under Soroban protocol compatibility matrix doc #84).

Files/Modules Likely Affected

• .github/workflows/ci.yml
• Cargo.toml (testing-only alt)

Difficulty Medium. Estimated Effort S.

Labels
area:ci, area:smart-contract-safety,
priority:medium

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H. Observability & Error Handling

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Imported from docs/open-issues-draft.md as draft issue #43 (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 implement it and open a PR referencing this issue shortly.

[Moonwalker-rgb]

Hi maintainer,

I would like to request assignment for this smart contract safety and CI ticket to add protocol minimum compatibility testing.

I will add a dedicated CI check in .github/workflows/ci.yml that compiles the smart contracts against the minimum protocol SDK version to prevent silent production deployment failures caused by unsupported host functions, and document the compatibility guidelines in docs/protocol-compatibility.md.

[grantfox-oss]

🦊 GrantFox — @Moonwalker-rgb has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #103)
2. Your PR will be reviewed by the StellarTips maintainers

Good luck! Track your progress on GrantFox.