Draft: Add Schnorr/FROST scaffold and tBTC runtime signing adapter slice

[mswilkison]

Summary

• wire build-tagged native FROST engine registration into the transitional native signing provider
• add a generic UniFFI native FROST signing engine adapter (uniFFINativeFROSTSigningEngine) with bridge abstraction and input/output validation
• add default frost_native registration fallback (no-op when UniFFI bridge is not compiled in)
• add optional UniFFI-backed registration path behind frost_native && frost_uniffi_sdk && cgo using github.com/zecdev/frost-uniffi-sdk/frost_go_ffi
• add tests for adapter behavior and optional tagged end-to-end signing/verification path
• thread canonical bridge wallet ID compatibility fields through tBTC chain/event models (WalletChainData, NewWalletRegisteredEvent) and map them in the Ethereum adapter via legacy-compatible derivation from wallet public key hash

Dependency Notes

• this branch currently uses internal integration pinning:
  • replace github.com/zecdev/frost-uniffi-sdk => github.com/tlabs-xyz/frost-uniffi-sdk v0.0.0-20260221162625-51e08b3fb886
• TODO after internal review: replace the fork pin with upstream zecdev/frost-uniffi-sdk commit/release once the upstream PR is opened and merged

Validation

• GOCACHE=/tmp/keep-core-gocache go test ./pkg/tbtc -run TestDeriveLegacyWalletID
• GOCACHE=/tmp/keep-core-gocache go test ./pkg/chain/ethereum -run TestCalculateWalletID
• GOCACHE=/tmp/keep-core-gocache go test -tags frost_native ./pkg/frost/signing
• GOCACHE=/tmp/keep-core-gocache go test -tags frost_native ./pkg/tbtc -run "TestConfigureFrostSigningBackend|TestNewNode_ConfiguresFrostSigningBackend|TestSigningExecutor_Sign_NativeBackend|TestSigningExecutor_Sign_FFIStrictBackend_WithNativeSignerMaterial|TestSigningExecutor_Sign_NativeBackend_FallsBackWhenOnlyLegacySignerMaterial"
• GOCACHE=/tmp/keep-core-gocache CGO_ENABLED=1 CGO_LDFLAGS='-lfrost_uniffi_sdk -L/Users/maclane/Projects/claude-test/frost-uniffi-sdk/target/debug -lm -ldl' LD_LIBRARY_PATH='/Users/maclane/Projects/claude-test/frost-uniffi-sdk/target/debug' go test -tags 'frost_native frost_uniffi_sdk' ./pkg/frost/signing -run TestBuildTaggedUniFFINativeFROSTBridge_EndToEndSigning

Notes

• Contributors: Threshold Labs
• This PR remains scaffold/integration focused; GG18 runtime replacement is not in scope here.