Scaffold flit: PWA + Go CLI for ephemeral E2E file transfer

Ports the yaw/2.1 identity/signaling/WebRTC transport from waste-go to
both a browser PWA (with QR pairing and Web Share Target) and a headless
Go CLI, trimmed to 1:1 ephemeral file transfer only.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pwa/src/pairing/keyring.ts

@@ -0,0 +1,28 @@
+// Persistent trusted-peer list ("Add device" flow) — separate from the
+// ephemeral one-shot QR room pairing in pairing/ephemeral.ts.
+
+export interface TrustedPeer {
+  id: string // hex Ed25519 pubkey
+  label: string
+  addedAt: number
+}
+
+const KEY = 'flit_keyring'
+
+export function listTrusted(): TrustedPeer[] {
+  try { return JSON.parse(localStorage.getItem(KEY) ?? '[]') } catch { return [] }
+}
+
+export function addTrusted(id: string, label: string): void {
+  const peers = listTrusted().filter(p => p.id !== id)
+  peers.push({ id, label, addedAt: Date.now() })
+  localStorage.setItem(KEY, JSON.stringify(peers))
+}
+
+export function removeTrusted(id: string): void {
+  localStorage.setItem(KEY, JSON.stringify(listTrusted().filter(p => p.id !== id)))
+}
+
+export function isTrusted(id: string): boolean {
+  return listTrusted().some(p => p.id === id)
+}