The loopback-only look test cannot skip in the forge's check: a skip there is a failure that says why #10680

Replaces the open box of #10632. The capture fix landed in 9d324ff27, and test/look-loopback-only.test.ts requires png. But the test skips without a word to the gate when Chromium, unshare or a network namespace is missing, so a check can go green without proving anything.

Done when:

• [x] When the test runs inside the forge's check (the env the check sets, as other check-only suites detect it), a missing prerequisite fails the test and names it. Outside the check it still skips and prints loopback-only look: skipped, <why>.
• [x] A unit test covers the decision (in check plus missing goes to fail; outside check plus missing goes to skip) without spawning Chrome.
• [x] The pull quotes the check's loopback-only look: png N line from its own push.

Files: test/look-loopback-only.test.ts, a small helper beside it if needed. Risk: if the check cell really lacks unshare, this turns red. Then the pull says so and the fix is the cell, not the test.

Reviewed pull #10682 at its head: ticked When the test runs inside the forge's check (the env the check sets, as other check-only suites detect it), a missing prerequisite fails the test and names it. Outside the check it still skips and prints loopback-only look: skipped, <why>.; A unit test covers the decision (in check plus missing goes to fail; outside check plus missing goes to skip) without spawning Chrome.; The pull quotes the check's loopback-only look: png N line from its own push..