Fix streaming and MCP reconnect races

[kylepelham]

Summary

• keep startup from creating a second global SSE event pump and duplicating streamed deltas
• reject stale MCP connection attempts before they can replace the active client, status, or tool cache
• add deterministic regression coverage for both lifecycle races

Validation

• bun run test (117 passed)
• bun run typecheck
• bun run test:engine
• cargo test --manifest-path src-tauri/Cargo.toml (17 passed)
• bun run build
• rebuilt the patched engine and produced a native NSIS package