feat(core): parse UdtStructV0 struct metadata from contract spec WASM

[Agbasimere]

Summary

• Parse ScSpecEntry::UdtStructV0 from contractspecv0 into cacheable ContractStructDef entries on ContractSpec (name, fields, ScSpecTypeDef).
• Keep nested/cyclic UDTs memory-safe: UDT names are not expanded during parse, XDR decode is bounded, and type formatting is depth-capped to avoid stack overflow.
• Cover nested structs, self-referential and mutually recursive UDTs, deep container nesting, and JSON roundtrip of the parsed spec.

Test plan

• cargo test -p grat-core --lib spec:: (27 passed)
• cargo fmt --all -- --check (currently fails on pre-existing files outside this change)
• cargo test --workspace --all-targets --all-features on Ubuntu CI (local MSVC SDK is missing kernel32.lib; multi_op_decoder also does not compile against stellar-xdr 21)
• Decode a contract return value that is a custom struct and confirm fields are labeled JSON

Closes: #395

[coderabbitai]

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[codeZe-us]

@Agbasimere PR reviewed and merged