[codex] Fix Firo Spark Name registration format

[reubenyap]

What changed

• Tags the Spark Name registration fee output as P2PKH + OP_SPARKNAMEID <name> OP_DROP <sparkAddress> OP_DROP when building Spark Name registration transactions.
• Reuses bitcoindart script compilation for canonical pushdata encoding.
• Bumps the Firo flutter_libsparkmobile dependency to firoorg/flutter_libsparkmobile@171bc186663e3c7a573a6240f28f430e8d6b7d50.

Why

Firo changed Spark Name registration transaction validation. Registration now needs the v2 Spark Name payload from the native library and a tagged transparent fee output so the fee output is bound to the registered name and Spark address.

Related

• Spark Mobile native PR: [codex] Update Spark Name registration payload format firoorg/sparkmobile#10
• Flutter wrapper PR: [codex] Update vendored Spark Name registration format firoorg/flutter_libsparkmobile#1

Review notes

This stays registration-only. Stack Wallet does not create Spark Name transfers, so transfer ownership fields/proofs are intentionally not added here.

Validation

• git diff --check passes locally, with only Windows CRLF warnings.
• Stale-symbol search found no leftover wrapper fee-script API.
• GitHub PR Test workflow passed: https://github.com/cypherstack/stack_wallet/actions/runs/28672496648/job/85038728700
• Manual upstream Build workflow dispatch was blocked by GitHub permissions (403 admin rights), and direct upstream branch push was denied (403), so I could not start the full multi-platform build workflow from here.
• The pinned firoorg/flutter_libsparkmobile native Windows CMake build was exercised from a temp clone and produced codex-build/windows/shared/libflutter_libsparkmobile.dll using local-only -DCMAKE_CXX_FLAGS=-fpermissive; the flag works around an existing vendored secp256k1/GCC 16 diagnostic, not this change.