fix: reject non-hex Stellar meta-address payloads

[xtrial01]

Summary

Closes #233 by rejecting Stellar meta-address payloads containing non-hexadecimal characters before decoding. Invalid payloads now produce a consistent validation error.

Added table-driven cases and a property test covering invalid characters across all payload positions.

Verification

• Stellar tests: 1,002 passed, 5 skipped.
• Package build passed.
• Prettier passed for both changed files.
• Full-repository checks have unrelated existing failures: formatting in seven other files, sdk-agent type errors, and workspace test failures involving missing Vue dependencies and compatibility documentation.

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[truthixify]

The decoder now rejects non-hex payloads before parsing and all 11 focused tests pass. Approved.