Remove unsupported UniFFI FROST material path

[mswilkison]

Summary

• Require the tbtc-signer engine for FROST DKG output and stop falling back to the generic UniFFI FROST DKG path
• Reject unsupported frost-uniffi-v2 signer material before signing, group-key extraction, Taproot output-key extraction, and FROST wallet-ID derivation
• Remove the UniFFI FROST wrapper adapters/tests while keeping the tbtc-signer ABI smoke coverage and legacy frost-uniffi-v1 ECDSA compatibility

Rationale

frost-uniffi-v2 is not a production compatibility format. It cannot produce Taproot-tweaked signatures, so a FROST wallet using it can accept Taproot deposits that are effectively unsweepable. This PR fails closed before new FROST wallet material or BTC deposits can be created on that unsupported path.

Tests

• go test -tags 'frost_native frost_tbtc_signer cgo' ./pkg/frost/signing
• KEEP_CORE_FROST_TBTC_SIGNER_ACCEPT_SCAFFOLD_KEY_GROUP=true go test -tags 'frost_native frost_tbtc_signer cgo' ./pkg/tbtc
• go test -tags 'frost_native frost_tbtc_signer frost_roast_retry cgo' ./pkg/frost/signing

Stacked on #4020.

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