Validate TOML schema version compatibility in StellarTomlParser #779

Description

StellarTomlParser parses stellar.toml files without checking the VERSION field. Future TOML schema versions may introduce breaking changes; parsing them with the current logic silently produces incorrect data.

Acceptance Criteria

• The parser reads the VERSION field from the TOML document
• Versions in SUPPORTED_TOML_VERSIONS (exported constant, e.g., [2.0, 2.1]) are accepted
• Unsupported versions throw a UnsupportedTomlVersionError naming the encountered version
• Unit tests pass

Context

• Target file: src/anchors/StellarTomlParser.ts
• The check should be the first validation after successful TOML parsing