docs(dx): add unified error code catalog with CI validation

[amosleoamadi]

Closes #168

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• Add stellar/ERRORS.md with a table per contract listing every ContractError variant:
  Code | Name | Meaning | Introduced in
  Cross‑reference each row to the line in source where the variant is defined.
• Add a Code allocation policy section that reserves disjoint numeric ranges per contract to prevent future collisions.
• Add a CI step (.github/workflows/ci.yml) that greps all ContractError enums in the source and fails if any variant is missing from ERRORS.md.
• Link to the catalog from stellar/README.md and from each contract’s README.md for discoverability.

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[truthixify]

Thanks @amosleoamadi. #169 just merged and it created the six per-contract READMEs that this PR adds cross-link lines to, so five files now conflict:

governance, stealth-announcer, stealth-batch-sender, stealth-vault, wraith-asset-policy READMEs.

When you rebase, keep both sides: their new README body plus your ERRORS.md cross-link line at the bottom.

git fetch origin
git rebase origin/develop

stellar/ERRORS.md and the CI validation step are clean, only the READMEs collide. The catalog check script itself looks good.

[truthixify]

Merged. Thanks for turning the rebase around @amosleoamadi, and for keeping both sides on the README conflicts rather than clobbering. The catalog check script is the durable part here.