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-contracts/src/mux/control.ts

@@ -0,0 +1,64 @@
+/**
+ * §4.1 control-frame payload codecs: WINDOW_UPDATE and GOAWAY.
+ *
+ * Per §4.1's explicit byte spec these are fixed-width big-endian integers (NOT CBOR):
+ *   WINDOW_UPDATE payload = uint32 credit
+ *   GOAWAY       payload = uint32 lastStreamId ‖ uint32 reason
+ * (PLAN_RELAY_INDEX §2.1 loosely groups them under "CBOR payload codecs"; §4.1 is the
+ *  authoritative wire and wins — see report deviation note.)
+ */
+import { ContractDecodeError } from '../errors.js'
+import { assertUint32, readUint32BE, writeUint32BE } from '../bytes.js'
+import {
+  GOAWAY_CODE_TO_REASON,
+  GOAWAY_REASON_TO_CODE,
+  type GoAwayReason,
+} from './types.js'
+
+/** Map a GOAWAY wire code to its frozen reason label; throws on unknown code. */
+export function decodeGoAwayReason(code: number): GoAwayReason {
+  const reason = GOAWAY_CODE_TO_REASON[code]
+  if (reason === undefined) {
+    throw new ContractDecodeError(`unknown GOAWAY reason code ${code}`)
+  }
+  return reason
+}
+
+/** WINDOW_UPDATE: encode a uint32 credit increment. */
+export function encodeWindowUpdate(credit: number): Uint8Array {
+  assertUint32(credit, 'credit')
+  const out = new Uint8Array(4)
+  writeUint32BE(out, 0, credit)
+  return out
+}
+
+/** WINDOW_UPDATE: decode the uint32 credit increment. */
+export function decodeWindowUpdate(payload: Uint8Array): number {
+  if (payload.length !== 4) {
+    throw new ContractDecodeError(`WINDOW_UPDATE payload must be 4 bytes; got ${payload.length}`)
+  }
+  return readUint32BE(payload, 0)
+}
+
+/** GOAWAY: encode uint32 lastStreamId ‖ uint32 reason-code. */
+export function encodeGoaway(lastStreamId: number, reason: GoAwayReason): Uint8Array {
+  assertUint32(lastStreamId, 'lastStreamId')
+  const code = GOAWAY_REASON_TO_CODE[reason]
+  if (code === undefined) {
+    throw new ContractDecodeError(`unknown GOAWAY reason label '${reason}'`)
+  }
+  const out = new Uint8Array(8)
+  let offset = writeUint32BE(out, 0, lastStreamId)
+  writeUint32BE(out, offset, code)
+  return out
+}
+
+/** GOAWAY: decode uint32 lastStreamId ‖ uint32 reason-code. */
+export function decodeGoaway(payload: Uint8Array): { lastStreamId: number; reason: GoAwayReason } {
+  if (payload.length !== 8) {
+    throw new ContractDecodeError(`GOAWAY payload must be 8 bytes; got ${payload.length}`)
+  }
+  const lastStreamId = readUint32BE(payload, 0)
+  const reason = decodeGoAwayReason(readUint32BE(payload, 4))
+  return { lastStreamId, reason }
+}