Inside a child cell, the looks browser cannot reach the child's own app (net::ERR_INTERNET_DISCONNECTED) #10557

On scottishbob's cell, every [looks] screenshot fails: no picture app=diagnostics env=production page=/ … net::ERR_INTERNET_DISCONNECTED. The cell's network is loopback only, and the screenshot browser tries the app's public URL. Point looks at the app's local port inside the cell (or through the cell's reach list), with a test that a cell child gets a picture of its own app. The cell must not gain any other network.

Parked by the crew: the check stayed red through 2 automatic tries (pull #10610 closed; its branch mason/10557-inside-a-child-cell-the-looks-browser-ca at 5bccf70a7c6a keeps the work so far). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the check stayed red through 2 automatic tries (pull #10615 closed; its branch scout/10557-inside-a-child-cell-the-looks-browser-ca at 013ad25c2a3b keeps the work so far).

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the check stayed red through 2 automatic tries (pull #10610 closed; its branch mason/10557-inside-a-child-cell-the-looks-browser-ca at 5bccf70a7c6a keeps the work so far)
2. the check stayed red through 2 automatic tries (pull #10615 closed; its branch scout/10557-inside-a-child-cell-the-looks-browser-ca at 013ad25c2a3b keeps the work so far)

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

Split by the planner into #10619 (the capture gets a picture on a loopback-only machine, tested in a network namespace) and #10620 (a cell child's looks picture its own app, cell-suite test, no new network). Both parked branches are gone from the forge, so the split starts fresh.