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/mux/flow-control.ts

@@ -0,0 +1,76 @@
+/**
+ * T4 · Credit-based per-stream flow control (§4.1) — pure, in-memory, deny-by-default.
+ *
+ * Each stream has an independent send window: exhausting stream A's credit MUST NOT affect
+ * stream B (heavy `vim`/`top` redraw can't starve another stream). Unknown/released streams
+ * carry no credit (deny-by-default: prevents send-after-close write amplification).
+ * Immutable-style updates (replace the record, never mutate in place).
+ */
+export const DEFAULT_INITIAL_WINDOW = 256 * 1024
+export const WINDOW_REPLENISH_THRESHOLD = 0.5 // emit WINDOW_UPDATE when half consumed
+
+interface StreamCredit {
+  readonly window: number // remaining SEND credit
+  readonly initial: number // initial window (for replenish threshold)
+  readonly delivered: number // RECEIVED bytes since last replenish
+}
+
+export interface FlowController {
+  registerStream(streamId: number, initialWindow: number): void
+  releaseStream(streamId: number): void
+  canSend(streamId: number, bytes: number): boolean
+  consumeSendCredit(streamId: number, bytes: number): void
+  grantCredit(streamId: number, credit: number): void
+  creditFor(streamId: number): number
+  onDelivered(streamId: number, bytes: number): number
+}
+
+export function createFlowController(): FlowController {
+  const streams = new Map<number, StreamCredit>()
+
+  return {
+    registerStream(streamId, initialWindow) {
+      streams.set(streamId, { window: initialWindow, initial: initialWindow, delivered: 0 })
+    },
+
+    releaseStream(streamId) {
+      streams.delete(streamId)
+    },
+
+    canSend(streamId, bytes) {
+      const c = streams.get(streamId)
+      if (c === undefined) return false // deny-by-default (unknown/released stream)
+      return bytes <= c.window
+    },
+
+    consumeSendCredit(streamId, bytes) {
+      const c = streams.get(streamId)
+      if (c === undefined) return
+      const nextWindow = c.window - bytes
+      // Never go negative (canSend gates callers; clamp defensively).
+      streams.set(streamId, { ...c, window: Math.max(0, nextWindow) })
+    },
+
+    grantCredit(streamId, credit) {
+      const c = streams.get(streamId)
+      if (c === undefined) return
+      streams.set(streamId, { ...c, window: c.window + credit })
+    },
+
+    creditFor(streamId) {
+      return streams.get(streamId)?.window ?? 0
+    },
+
+    onDelivered(streamId, bytes) {
+      const c = streams.get(streamId)
+      if (c === undefined) return 0
+      const delivered = c.delivered + bytes
+      if (delivered >= c.initial * WINDOW_REPLENISH_THRESHOLD) {
+        streams.set(streamId, { ...c, delivered: 0 })
+        return delivered // replenish this many bytes of credit back to the peer
+      }
+      streams.set(streamId, { ...c, delivered })
+      return 0
+    },
+  }
+}