test(pr3866): in-process coordinator↔chain FROST DKG end-to-end

[mswilkison]

What this adds

An in-process integration test that chains the full FROST wallet-creation
coordinator↔chain flow into ONE run:

local chain emits FrostDKGStarted
  → initializeFrostDKGCoordinator's OnFrostDKGStarted subscription fires
    → handleFrostDKGStarted (dedup → confirm block → GetFrostDKGState →
       PastFrostDKGStartedEvents → resolve group membership)
      → executeFrostDKGIfPossible (readiness announcement → REAL cgo DKG via
         executeFrostDKG/RunDKGWithSeed → signer registration → result assembly
         → DKG-result operator-signature collection)
        → SubmitFrostDKGResult (through the FrostDKGChain interface)
          → wallet verified registered on the local chain

New file: pkg/tbtc/frost_dkg_coordinator_chain_e2e_frost_native_test.go
(build-gated frost_native && frost_tbtc_signer && cgo).

Why

Today the coordinator↔chain wiring and the real cgo DKG execution are tested
separately and never in one flow:

• frost_dkg_coordinator_test.go drives the chain plumbing with stub results.
• frost_dkg_execution_frost_native_test.go drives the real DKG in isolation.

This closes that gap: event delivery, confirmation, membership resolution,
readiness announcement, result assembly/signature collection, on-chain
submission, and wallet registration all run together against a real cgo DKG
output.

What is REAL vs REDUCED

Real

• The DKG output. executeFrostDKG calls the process-global cgo
  tbtc-signer engine (buildTaggedTBTCSignerEngine, registered via
  RegisterNativeExecutionFFISigningPrimitiveForBuild). The x-only group key
  that lands on-chain is the exact key the engine produced.
• The submission path. The result is submitted through the FrostDKGChain
  interface (SubmitFrostDKGResult) — not injected into chain state directly.
• The coordinator wiring. The event is delivered through the
  OnFrostDKGStarted subscription registered by
  initializeFrostDKGCoordinator; confirmation, state check, past-event lookup,
  membership resolution, readiness announcement, DKG-result operator-signature
  collection, and delayed submission all run as in production.

Reduced (documented)

• Group size / custody. The group is a 3-seat group whose 3 seats are all
  held by ONE operator/node. The cgo engine is a process-global
  OnceLock<Mutex>, so N independent real-custody participants cannot run
  concurrently in one OS process. The tbtc-signer development dealer DKG by
  design has a single engine hold every participant's key package — which is
  exactly this shape. The cgo library rejects n == 1
  (participants must contain at least 2 entries), so the minimum honest
  reduction is n >= 2; the test uses GroupSize=3, GroupQuorum=2, HonestThreshold=2.
• Signer profile. The test sets TBTC_SIGNER_PROFILE=development (plus a
  hermetic state-encryption key + per-process state path, mirroring the existing
  real-cgo reference harness). Bootstrap/dealer DKG is disabled under the
  production profile, which requires distributed DKG wiring across processes.

Nothing about the crypto or the submission is faked: the DKG output comes from
the real engine and the submission goes through the chain interface.

Load-bearing assertion

A thin recording wrapper delegates every engine method to the real cgo engine
and captures the exact x-only key returned by RunDKGWithSeed. The test then
asserts the on-chain submitted Result.XOnlyOutputKey:

1. equals the captured real engine output byte-for-byte (so no injected /
   fake result can pass),
2. is non-zero and lifts to a valid secp256k1 curve point, and
3. the wallet derived from it is registered on the local chain
   (IsFrostWalletRegistered).

How to run

export FROST_LIB_DIR=<path to libfrost_tbtc dir>
export CGO_ENABLED=1
export CGO_LDFLAGS="-L${FROST_LIB_DIR} -Wl,-rpath,${FROST_LIB_DIR} -lfrost_tbtc"
export KEEP_CORE_FROST_REQUIRE_CGO=true
go test -tags "frost_native frost_tbtc_signer cgo" -count=1 -v \
  -run 'TestFrostDKGCoordinatorChainEndToEnd_RealCgo' ./pkg/tbtc/

Proof lines from a passing run:

STEP 2: emitting FrostDKGStarted seed=0x42ef0705...
STEP 3+4: chain received SubmitFrostDKGResult x-only=3971f8481d56...d03d wallet=00...453a
STEP 4: SubmitFrostDKGResult observed on-chain
LOAD-BEARING: real cgo DKG x-only key 3971f8481d56...d03d landed on-chain via the coordinator
STEP 5: wallet 00...453a registered on-chain
--- PASS: TestFrostDKGCoordinatorChainEndToEnd_RealCgo

Without the cgo lib linked the test skips (or fails when
KEEP_CORE_FROST_REQUIRE_CGO=true), so it stays inert where real crypto is
unavailable. It is excluded from the plain frost_native build, so the existing
coordinator tests are unaffected (go test -tags frost_native ./pkg/tbtc/
stays green).

Not covered / follow-up

This is an in-process rehearsal with one node holding all seats. It does NOT
cover the fully-live path:

• N distinct node processes each holding real per-seat custody, running the
  production distributed DKG (not the development dealer DKG), which is what
  TBTC_SIGNER_PROFILE=production requires.
• A real chain (hardhat / Ethereum) with staked operators and sortition
  emitting DkgStarted, real SelectFrostGroup, and on-chain result
  validation / challenge / approve, instead of the test-local FrostDKGChain.
• DKG-result signature collection across multiple operators over a live
  network (here a single operator's signature satisfies the reduced group's
  threshold with no network round-trip).

Those belong in a multi-process / system-test rehearsal and are out of scope for
this in-process integration test.

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