[codex] Update vendored Spark Name registration format

[reubenyap]

What changed

• Updates the vendored sparkmobile Spark Name transaction data struct to v2 registration format.
• Adds the v2 operationType byte, defaulting to registration (0).
• Extends the vendored Spark Name test to assert v2 registration serialization.

Why

Stack Wallet consumes Spark Name registration through this Flutter wrapper. The native Spark Name registration payload changed upstream, so the vendored copy needs the same minimal registration-format update.

Related

• Native Spark Mobile PR: [codex] Update Spark Name registration payload format sparkmobile#10

Review notes

No new Dart or FFI API is added. Stack builds the fee-output script itself; this wrapper change only updates the serialized Spark Name payload.

Validation

• git diff --check passes locally, with only Windows CRLF warnings.
• Windows native CMake/Ninja build succeeded from a temp clone with MSYS2, producing codex-build/windows/shared/libflutter_libsparkmobile.dll.
• The successful build used local-only -DCMAKE_CXX_FLAGS=-fpermissive after strict GCC 16 stopped in existing vendored secp256k1 extern/static declarations. No repository build flags were changed.
• Dart/Flutter plugin tests were not run locally because the Dart/Flutter SDK is not installed in this environment.