Onboarding core: setup payloads, enrollment tickets, scan-to-join (docs/10)

* kv-proto: ticket and join wire types, LoginFinish.ticket, b64_opt.
* kv-crypto: pairing.rs (HPKE AuthPSK seal/open with the QR secret as PSK, SAS code).
* kv-sync: onboarding.rs (kestrove://setup/v1 and kestrove://join/v1 codec with
  strict validation), ApiClient ticket/join calls and login_with_ticket, test
  server support for tickets and rendezvous.
* kv-ffi: parse/encode_setup_payload, setup_payload_for_kit, add_device_payload,
  revoke_enrollment_ticket, login_with_setup, join_start/poll/cancel,
  join_approve/send. The Secret Key is kept locally, HPKE-sealed to enc_pk.
* Tests: codec round trips and rejections, AuthPSK tamper cases, end-to-end
  Add-device QR and scan-to-join between two clients, forged bundles, bound
  tickets; live test extended with scan-to-join.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

Cargo.lock

@@ -1347,6 +1347,7 @@ dependencies = [
 name = "kv-ffi"
 version = "0.1.0"
 dependencies = [
+ "base64 0.22.1",
  "ciborium",
  "hkdf",
  "kv-crypto",