[codex] Update Spark Name registration payload format

[reubenyap]

What changed

• Bumps CSparkNameTxData::CURRENT_VERSION from 1 to 2.
• Adds the v2 operationType byte, defaulting to 0 for registration.
• Extends the Spark Name unit test to assert v2 registration serialization.

Why

Firo's Spark Name registration format now serializes registration data as v2 and includes an operation type. Stack Wallet only creates registrations, so this intentionally keeps transfer fields out of the mobile library.

Review notes

This is the smallest native change needed for registration compatibility. Transfer support can be added later if mobile starts creating Spark Name transfers.

Validation

• git diff --check passes locally, with only Windows CRLF warnings.
• Attempted spark_name_tests from a temp clone with MSYS2/UCRT; secp256k1 built, then the standalone compile stopped on missing endian.h because this repo's bare test target assumes POSIX/Linux headers.
• Attempted MSYS compile-only, but mixing the MSYS compiler with UCRT Boost/OpenSSL headers caused target-header conflicts; I stopped there rather than patching build files for a local-only setup.
• The same Spark Name source compiled successfully through firoorg/flutter_libsparkmobile's Windows CMake build and linked libflutter_libsparkmobile.dll.