test(e2e): enable Canton signature verification in devstack and add signed registration tests

## Background

Canton native user registration requires a valid Canton cryptographic signature to prove ownership of the party key. The api-server validates this via auth.VerifyCantonSignature (pkg/config/config.go: SkipCantonSigVerify).

The E2E devstack currently bypasses this check by forcing SKIP_CANTON_SIG_VERIFY=true (set in tests/e2e/cmd/devstack/main.go and tests/e2e/devstack/docker/compose.go). The existing TestRegister_CantonNative_* tests run with signature verification disabled — the happy path with a real signature is untested in CI.

## What Needs to Be Done

### 1. Wallet signing helper in the E2E devstack

Add a utility that produces a valid Canton signature for a given party and message. Canton uses Ed25519 keys managed by the participant node. The devstack already allocates parties via AllocateParty; it needs to expose signing too.

Options to explore:
- Canton admin API — check if ParticipantAdminService exposes a SignBytes or equivalent RPC
- Canton console script — run a one-shot canton script that signs a message and returns the signature
- Deterministic test key — provision the devstack with a known Ed25519 keypair for a dedicated test party and embed it in tests/e2e/devstack/util/

Target DSL interface:

sig, err := sys.Canton.SignMessage(ctx, partyID, message)

### 2. New E2E test: Canton native registration with real signature

Once the signing helper exists, add a test that exercises the full verified flow with SKIP_CANTON_SIG_VERIFY=false.

### 3. Devstack configuration

Support running the api-server in two modes: with and without sig verification. This could be done by parameterising NewAPIStack(t) with an options struct, or by running a second api-server container with SKIP_CANTON_SIG_VERIFY=false.

## Acceptance Criteria

- [ ] tests/e2e/devstack/ has a Canton.SignMessage(ctx, partyID, message) helper returning a valid Canton signature
- [ ] At least one E2E test exercises Canton native registration with signature verification enabled
- [ ] Existing TestRegister_CantonNative_* tests continue to pass (bypass still available for isolation)
- [ ] SKIP_CANTON_SIG_VERIFY documented in devstack comments explaining when each mode applies