feat: add CI gate for contract ABI compatibility

[Cerome360]

Closes #111

Summary

This PR adds a CI workflow step to prevent accidental and silent changes to contract ABIs, which can break downstream indexers and SDKs.

What Changed

• Added a new bash script stellar/scripts/generate-abis.sh to extract the ABI for the four main smart contracts using stellar contract inspect.
• Added the abi:snapshot script to stellar/package.json for easy updates.
• Captured the initial valid ABI snapshots for stealth_announcer, stealth_registry, stealth_sender, and wraith_names under stellar/abi/.
• Updated .github/workflows/ci.yml to generate the ABIs during the stellar CI job and fail if they differ from the committed snapshots using git diff --exit-code abi/.
• Included a README in stellar/abi/ to instruct contributors on how to intentionally update the ABI.

Design Decisions & Notes

• Used stellar contract inspect as specified by the issue.
• Security Note: The ABI generator reads local artifacts and operates purely in CI, with no reliance on external client inputs or secrets.

Acceptance Criteria

• Snapshot lands committed for all four contracts
• PR that changes a signature without updating snapshot fails CI
• stellar/abi/README.md explains the flow

[truthixify]

Rebase against develop so the updated CI workflow picks up this branch. Right now no full CI run has fired yet.

[truthixify]

Conflicts against develop from the recent merge batch. Rebase and force-push.