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.

term-relay/data-plane/subdomain-router.ts

@@ -0,0 +1,43 @@
+/**
+ * T7 · Subdomain router (§4.2 routing). `extractSubdomain` returns the SINGLE leftmost label
+ * of `host` under `baseDomain`, or null for a bare/nested/confusing Host (INV1 Host-confusion
+ * guard). It is a HINT for candidate lookup only — NEVER the authority: T8 requires the
+ * capability token's `aud` to equal this subdomain and P5 gates `token.host` against the
+ * resolver's `hostId`. `RouteResolver` is IMPLEMENTED BY P3; P1 owns only the interface.
+ */
+export interface ResolvedHost {
+  readonly hostId: string
+  readonly accountId: string
+  readonly subdomain: string
+}
+
+export interface RouteResolver {
+  resolveSubdomain(subdomain: string): Promise<ResolvedHost | null>
+}
+
+const LABEL_RE = /^[a-z0-9]([a-z0-9-]*[a-z0-9])?$/
+
+/** Strip a single trailing dot and lowercase; strip a `:port` suffix if present. */
+function normalizeHost(host: string): string {
+  const noPort = host.split(':', 1)[0] ?? host
+  const trimmed = noPort.endsWith('.') ? noPort.slice(0, -1) : noPort
+  return trimmed.toLowerCase()
+}
+
+/**
+ * Extract the single tenant label: `'alice.term.example.com'` under `'term.example.com'` → `'alice'`.
+ * Returns null when: host equals the base domain (bare), the suffix does not match, or the prefix
+ * is multi-label (`'a.b.term.example.com'` → null — nested subdomains are rejected, INV1).
+ */
+export function extractSubdomain(hostHeader: string, baseDomain: string): string | null {
+  const host = normalizeHost(hostHeader)
+  const base = normalizeHost(baseDomain)
+  if (host === base) return null // bare base domain — no tenant
+  const suffix = '.' + base
+  if (!host.endsWith(suffix)) return null // confusion attempt (e.g. '...example.com.evil.com')
+  const label = host.slice(0, host.length - suffix.length)
+  if (label.length === 0) return null
+  if (label.includes('.')) return null // multi-label prefix rejected (single-label tenant only)
+  if (!LABEL_RE.test(label)) return null // invalid DNS label
+  return label
+}