fix: enforce WebAuthn assertion type and User-Present flag

[alberto-crossmint]

Summary

The WebAuthn verifier (auth/signers/webauthn.rs) was missing two spec-mandated checks:

Spec	Check	Fix
WebAuthn §7.2.11	clientDataJSON.type == "webauthn.get"	Add ty field to ClientDataJson and reject anything other than "webauthn.get"
WebAuthn §6.1	User-Present (UP) bit set in authenticator_data flags	Reject if authenticator_data[32] & 0x01 == 0

Both checks are cheap (one string compare, one bit test) and live in the existing fail-fast block ahead of the ECDSA verification.

Why this matters

Type check: A signature produced during a navigator.credentials.create() ceremony is signed over a clientDataJSON whose type is "webauthn.create". Without the type check, that signature could be replayed against this contract if its challenge happened to match a Stellar transaction hash. The attacker scenario requires luring the victim into a registration ceremony on a hostile origin, but the defense is essentially free and removes a class of cross-ceremony replay risk that the WebAuthn spec specifically calls out.

UP flag: Modern authenticators almost always set the UP bit, but the spec is clear that the verifier must require it — without enforcement the contract trusts whatever the authenticator reports, including assertions that occurred without any user interaction.

Test plan

• test_webauthn_wrong_type_rejected — assertion with type: "webauthn.create" returns Err(InvalidWebauthnClientDataJson).
• test_webauthn_wrong_type_rejected_end_to_end — same, through try_invoke_contract_check_auth.
• test_webauthn_missing_user_present_flag_rejected — authenticator_data[32] = 0x00 returns Err(InvalidWebauthnClientDataJson).
• All existing webauthn tests still pass — the existing test_utils::WebauthnTestSigner already produces type: "webauthn.get" and flags: 0x01 so no regression.
• Full suite: 129 passed, 0 failed (3 new tests, no regressions).

Out of scope

• clientDataJSON.origin and authenticator_data rpIdHash enforcement. Both require a per-signer expected value (origin string / RP ID), which means a WebauthnSigner schema change and migration. Tracked for a future v3 schema bump.
• Counter monotonicity. WebAuthn lets the RP enforce it; in our model each transaction signs a fresh challenge so replay isn't possible without a counter, but spec compliance would add it. Tracked separately.

Backwards compatibility

Real-world authenticators producing type: "webauthn.get" with UP set (overwhelming majority) are unaffected. Any client previously relying on the lax verification behavior — sending non-"webauthn.get" ceremonies or zero flags — will now be rejected. A grep across known integrations is suggested before rollout.