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/transcript.ts

@@ -0,0 +1,41 @@
+/**
+ * Deterministic handshake transcript builder. Both endpoints concatenate the SAME length-prefixed
+ * fields in the SAME fixed order (client_hello fields, then host_hello fields EXCLUDING `sig`), so
+ * the transcript the host signs is byte-identical to the one the client verifies. `deviceAuthProof`
+ * is excluded (it is verified separately and is not part of the signed key-agreement transcript).
+ */
+import type { AeadAlg } from 'relay-contracts'
+import { concatBytes, writeUint32BE } from 'relay-contracts'
+
+const encoder = new TextEncoder()
+const DOMAIN = encoder.encode('relay-e2e/transcript/v1')
+
+function lp(chunk: Uint8Array): Uint8Array {
+  const head = new Uint8Array(4)
+  writeUint32BE(head, 0, chunk.length)
+  return concatBytes([head, chunk])
+}
+
+export interface TranscriptParts {
+  readonly clientEphPub: Uint8Array
+  readonly clientNonce: Uint8Array
+  readonly aeadOffer: readonly AeadAlg[]
+  readonly hostEphPub: Uint8Array
+  readonly hostNonce: Uint8Array
+  readonly aeadChoice: AeadAlg
+  readonly enrollFpr: string
+}
+
+/** Build the byte-identical transcript both sides bind the Ed25519 signature to. */
+export function buildTranscript(p: TranscriptParts): Uint8Array {
+  return concatBytes([
+    lp(DOMAIN),
+    lp(p.clientEphPub),
+    lp(p.clientNonce),
+    lp(encoder.encode(p.aeadOffer.join(','))),
+    lp(p.hostEphPub),
+    lp(p.hostNonce),
+    lp(encoder.encode(p.aeadChoice)),
+    lp(encoder.encode(p.enrollFpr)),
+  ])
+}