Register tbtc-signer interactive FROST engine

[mswilkison]

Summary

• register the build-tagged tbtc-signer bridge as the native FROST DKG engine and the full native FROST signing engine
• add Go request/response adapters for the new interactive Rust ABI from Expose interactive FROST DKG signer ABI #4011
• keep old/missing signer builds fail-closed through ErrNativeCryptographyUnavailable
• disable legacy ECDSA DKG execution when the configured pre-parameters pool size is zero, so FROST-only local/testnet runs do not require an unused ECDSA preparams pipeline
• reject Schnorr/FROST signing executors for non-Taproot transaction inputs before threshold signing begins
• add registration coverage plus an optional linked-dylib smoke test that runs DKG, signing, aggregation, and BIP340 verification when the Rust symbols are present

Stack

• base: Route FROST authorization through registry #4010
• requires Rust signer ABI: Expose interactive FROST DKG signer ABI #4011

Testnet rehearsal note

The local FROST testnet reached threshold signing for a real revealed Bitcoin testnet deposit and produced a native aggregate signature. The action then failed while applying the signature because the deposit sweep input was a legacy P2WSH deposit output requiring ECDSA semantics, not a Taproot key-path wallet input.

This PR now fails closed earlier for that class of mismatch: a Schnorr/FROST signing executor will not sign non-P2TR transaction inputs. This does not make legacy deposits sweepable by FROST; it prevents wasting a threshold signing round and producing a misleading invalid-signature failure.

Validation

• go test -tags 'frost_native frost_tbtc_signer' ./pkg/frost/signing
• CGO_LDFLAGS='-L/private/tmp/keep-core-pr4005-worktree/pkg/tbtc/signer/target/debug -lfrost_tbtc' DYLD_LIBRARY_PATH=/private/tmp/keep-core-pr4005-worktree/pkg/tbtc/signer/target/debug go test -tags 'frost_native frost_tbtc_signer' ./pkg/frost/signing -run TestBuildTaggedTBTCSignerInteractiveFROSTBridge_WithLinkedSigner -count=1 -v
• KEEP_CORE_FROST_TBTC_SIGNER_ACCEPT_SCAFFOLD_KEY_GROUP=true go test -tags 'frost_native frost_tbtc_signer' ./pkg/tbtc
• go test ./pkg/tbtc -run 'TestWalletTransactionExecutor_SignTransaction_RejectsSchnorrForLegacyInputsBeforeSigning|TestWalletTransactionExecutor_SignTransaction_RejectsMixedTaprootAndLegacyInputsBeforeSigning|TestWalletTransactionExecutor_SignTransaction_AddsTaprootKeyPathWitness|TestDkgExecutor_DisablesECDSAPreParamsWhenPoolSizeZero'
• go test -tags frost_native ./pkg/tbtc -run 'TestWalletTransactionExecutor_SignTransaction_RejectsSchnorrForLegacyInputsBeforeSigning|TestWalletTransactionExecutor_SignTransaction_RejectsMixedTaprootAndLegacyInputsBeforeSigning|TestWalletTransactionExecutor_SignTransaction_AddsTaprootKeyPathWitness|TestDkgExecutor_DisablesECDSAPreParamsWhenPoolSizeZero|TestCalculateWalletIDForSigner_FrostUniFFIV2UsesXOnlyOutputKey'

Note: without KEEP_CORE_FROST_TBTC_SIGNER_ACCEPT_SCAFFOLD_KEY_GROUP=true, the broad pkg/tbtc run fails on existing scaffold-era legacy private-key-share fixtures, not on this bridge registration change.

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