feat(relay): rendezvous-relay service — 7 packages + plans (contracts/transport/agent/control-plane/e2e/auth/web)

Multi-tenant reverse-tunnel service ("ngrok for Claude Code" with E2E): a
host-agent dials OUT to an operator-run relay; external devices reach the host
THROUGH the relay, routed by per-tenant subdomain, forwarding ciphertext only
(the relay never sees plaintext). Lets a customer reach their own self-hosted
web-terminal from anywhere with zero networking setup.

Packages — all tsc-strict + vitest green (656 tests), cross-package integration verified:
- relay-contracts: frozen shared contracts (mux frame codec, data model,
  capability token, E2E envelope, pairing) — the src/types.ts analog
- term-relay:   native WS mux + stateless data plane (subdomain routing, ciphertext forward)
- agent:        host-agent (pairing, per-host Ed25519 + mTLS dial-out, forwards to 127.0.0.1:3000)
- control-plane: accounts/hosts registry, pairing-code flow, routing table, provisioning
- relay-e2e:    browser<->agent E2E (X25519 ECDH through relay, AEAD, anti-replay, recoverable replay key)
- relay-auth:   Passkey/WebAuthn, capability tokens, per-host certs, deny-by-default tenant isolation
- relay-web:    browser login + Web Crypto E2E + client-side preview rendering

Security invariants INV1-15 enforced; cross-tenant isolation CI tripwire live
(.github/workflows/relay-tripwire.yml). Design + implementation-level plans in
docs/PLAN_RELAY_*.md and docs/EXPLORE_RELAY_SERVICE.md.

NOTE: generated autonomously per the reviewed plans. The security-critical
packages (relay-e2e, relay-auth) REQUIRE expert security audit before any real
deployment — passing tests prove self-consistency, not resistance to attackers.
Base app (src/, public/) unchanged; concurrent desktop work left uncommitted.

relay-e2e/src/envelope.ts

@@ -0,0 +1,49 @@
+/**
+ * T3 — E2EEnvelope wire codec.
+ *
+ * RECONCILIATION: the frozen `relay-contracts` §4.4 surface ALREADY owns the envelope binary
+ * layout and the deterministic per-seq nonce (`encodeEnvelope`/`decodeEnvelope`/`nonceForSeq`,
+ * 14-byte header: seq(8)‖nonceLen(1)‖ciphertextLen(4)‖tagLen(1) then nonce‖ciphertext‖tag).
+ * The plan's stale T3 byte layout (envVersion/aeadId) is SUPERSEDED — relay-contracts owns the
+ * shape (INDEX §2.1), so P4 CONSUMES/RE-EXPORTS it and never redefines it. This module only adds
+ * a boundary guard that re-throws the contract's decode error as a P4-typed `EnvelopeFormatError`
+ * and enforces a hard frame-size cap (anti-overrun; the alg is carried by the key/session, not the
+ * wire, so there is no unchecked wire-length allocation).
+ */
+import type { E2EEnvelope } from 'relay-contracts'
+import {
+  ContractDecodeError,
+  decodeEnvelope as decodeEnvelopeRaw,
+  encodeEnvelope as encodeEnvelopeRaw,
+  nonceForSeq,
+} from 'relay-contracts'
+import { EnvelopeFormatError } from './errors.js'
+
+/** Hard upper bound on a decoded frame (guards a decompression-bomb-style length overrun). */
+export const MAX_FRAME_BYTES = 4 * 1024 * 1024
+
+export { nonceForSeq }
+
+/** Encode a §4.4 E2EEnvelope to its frozen wire bytes (delegates to relay-contracts). */
+export function encodeEnvelope(env: E2EEnvelope): Uint8Array {
+  const bytes = encodeEnvelopeRaw(env)
+  if (bytes.length > MAX_FRAME_BYTES) {
+    throw new EnvelopeFormatError(`encoded frame ${bytes.length} exceeds cap ${MAX_FRAME_BYTES}`)
+  }
+  return bytes
+}
+
+/** Decode wire bytes into a §4.4 E2EEnvelope; malformed input ⇒ typed `EnvelopeFormatError`. */
+export function decodeEnvelope(buf: Uint8Array): E2EEnvelope {
+  if (buf.length > MAX_FRAME_BYTES) {
+    throw new EnvelopeFormatError(`frame ${buf.length} exceeds cap ${MAX_FRAME_BYTES}`)
+  }
+  try {
+    return decodeEnvelopeRaw(buf)
+  } catch (err) {
+    if (err instanceof ContractDecodeError) {
+      throw new EnvelopeFormatError(err.message)
+    }
+    throw err
+  }
+}