Feature: Expanded store-and-forward (open couriers, durable outbox, 6h public history)

Summary

Port iOS expanded store-and-forward so DMs and public history eventually deliver through crowds of strangers — not only mutual favorites currently connected.

Android has a basic StoreForwardManager; iOS substantially extended the model.

Upstream (iOS)

• https://github.com/permissionlesstech/bitchat/pull/1372 — Expand S&F: open couriers, spray-and-wait, persistent outbox, 6h public history
• https://github.com/permissionlesstech/bitchat/pull/1415 — DMs to unreachable peers use S&F instead of failing instantly
• https://github.com/permissionlesstech/bitchat/pull/1368 — Courier deposit in parallel when only route is a send round
• Bridge integration: https://github.com/permissionlesstech/bitchat/pull/1412

What it is

• MessageOutboxStore: queued private messages persist to disk (sealed); retry across app restarts
• Open couriers: strangers can carry sealed mail (quota-bounded), not only mutual favorites at send time
• Spray-and-wait: multi-carrier deposit strategy
• 6h public history persistence for peer serve + echoes UX
• No instant fail when peer is neither mesh-reachable nor Nostr-mutual — hand to router/outbox/courier/bridge

Why Android needs it

Delivery parity is fundamental; without it Android DMs still die when the recipient briefly leaves range, while iOS queues them.

Acceptance criteria

• Durable encrypted outbox survives process death
• Open-courier deposit/pickup wire-compatible with iOS
• Unreachable DM path never marks failed without enqueue
• Public history retention window parity (~6h) for sync/echoes
• Quotas, expiry, panic-wipe behavior

Priority

High — core messaging reliability; prerequisite for bridge/courier excellence.