The loopback-only looks test runs in the forge's check and says what Chrome does there #10631

First half of #10619 (parked twice: pull #10625 guessed a fix nobody saw fail). Prove the failure before fixing it. No change to src/looks/capture.ts.

Done when:

• [x] test/look-loopback-only.test.ts (take it from the branch of pull #10625) runs takeLook inside unshare -rn with only lo up, against a Bun server on 127.0.0.1, and prints one line: loopback-only look: png <bytes> or loopback-only look: error <message>.
• [x] It asserts only that one of the two came back (never hangs: a 30 s budget, past it a named failure), so it is green on today's main whichever happens.
• [ ] It skips by name, the reason printed, where unshare, ip, Chromium or namespace permission is missing; the pull says the check log line it produced (ran, not skipped) and what it printed.
• [x] The file is in the gate list if the gate needs one; bun test test/look-refusal.test.ts still passes.

Files: test/look-loopback-only.test.ts, test/look-loopback-only.child.ts. Risk: the builder's sandbox cannot make a namespace; the check's log is the evidence.

Reviewed pull #10633 at its head: ticked test/look-loopback-only.test.ts (take it from the branch of pull #10625) runs takeLook inside unshare -rn with only lo up, against a Bun server on 127.0.0.1, and prints one line: loopback-only look: png <bytes> or loopback-only look: error <message>.; It asserts only that one of the two came back (never hangs: a 30 s budget, past it a named failure), so it is green on today's main whichever happens.; The file is in the gate list if the gate needs one; bun test test/look-refusal.test.ts still passes.. left open: It skips by name, the reason printed, where unshare, ip, Chromium or namespace permission is missing; the pull says the check log line it produced (ran, not skipped) and what it printed. — The skip logic is right, but the pull does not yet name the check log line; the builder has only seen the skip in their own sandbox..

Left open after pull #10633 landed: 1 unticked item remains (It skips by name, the reason printed, where unshare, ip, Chromium or namespace permission is missing; the pull says the check log line it produced (ran, not skipped) and what it printed.). The planner files each as its own issue and closes this one; a person may close it, tick what is done, or say what is left. Until then the crew does not pick it again.

Dropped: the probe landed in #10633. The remaining box only asked the pull to quote its check log line, and #10632 now starts by reading that line from the check log. Nothing is left to build here.

Deferred behind the Later gate: there's nothing left to build. On main the probe already skips by name and prints the reason (test/look-loopback-only.test.ts:30). The quoted log line is now the first step of #10632, which no longer waits on this issue. The forge refused both drop and done, so a person can close this.