Improve iPhone↔Mac connection reliability

[donny-son]

Why

The MultipeerConnectivity link between the iPhone and Mac was unstable — commands occasionally dropped and reconnects were unreliable. Reviewing both connection managers surfaced several concrete causes.

What changed

1. Mac recreates its session on disconnect (biggest fix)
Previously the iPhone recreated its MCSession on every drop but the Mac reused its old one for the next invitation. After a drop that session can be left half-dead, so the next connection was flaky. The Mac now recreates a clean session on full disconnect, mirroring the iPhone.

2. Bidirectional keepalive + stale-link watchdog
The keepalive was fire-and-forget (iPhone → Mac, no reply), so neither side could tell a "connected" session had actually gone dead. Now:

• The Mac replies to each keepalive with a keepaliveAck.
• Both sides track the time of the last inbound packet and, if nothing arrives for ~9s (≈3 missed keepalives), tear down and cleanly reconnect.

This catches half-open links that MCSession still reports as .connected — the usual cause of silently-dropped slide commands.

3. Duplicate-invitation guard
The reconnect timer and browser(foundPeer:) could both invite the same Mac at once, which makes MultipeerConnectivity bounce the connection. Invitations are now guarded by an in-flight flag.

4. Persistent MCPeerID
Both apps created a fresh MCPeerID each launch. Apple recommends reusing one — a new ID with the same display name is treated as a different peer, causing phantom/duplicate peers and confusing reconnection-by-name. It's now archived to UserDefaults and reused.

5. Reconnect backoff + faster invite timeout
Fixed 1s retry replaced with exponential backoff + jitter (1→2→4→8s capped). Invite timeout shortened 30s → 15s so a stuck invitation no longer blocks reconnection for half a minute.

Notes

• The persistent-peer helper is wrapped in #if canImport(MultipeerConnectivity) because Shared/ also compiles into the watchOS target, where the framework isn't available.
• No exhaustive switch over RemoteCommand exists outside the enum itself, which was updated for the new keepaliveAck case.

Testing

⚠️ Built and reviewed on Linux — not compiled against the iOS/macOS SDKs, which aren't available in this environment. Please build the DeckMac and DeckiOS schemes locally and verify a real iPhone↔Mac connect / drop / reconnect cycle before merging.

🤖 Generated with Claude Code

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Generated by Claude Code