Support Taproot tweaked signer rounds

[mswilkison]

Stacked on extraction/frost-signer-mirror-2026-05-26 / PR #4005. Adds optional taproot_merkle_root_hex to start/finalize signing rounds, binds it into request fingerprints and round IDs, signs and aggregates with frost-secp256k1-tr Taproot tweaks, and verifies tweaked aggregates in tests. Verification: cargo test in pkg/tbtc/signer.

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