The gate is every test/*.test.ts minus EXCLUDED: GATE is deleted and a pull adding a test touches no shared file #2071

Split from #2037 (a person's ask). Pull #2060's first commit (9d13a4f7, branch of #2060) already does this: gateFiles() = presentTests() minus EXCLUDED, in name order. The audit checks only exclusions, and callers (scripts/test-shards.ts, scripts/flake-hunt.ts, scripts/flake.ts quarantine, gateListAdds in src/checks/impact.ts) follow. Start from it.

Done looks like:

• [ ] GATE is gone from scripts/gate.ts; EXCLUDED is the reasoned quarantine, each line naming its issue.
• [ ] test/check-list.test.ts and the impact map follow. A test shows that a tree which only adds test/x.test.ts gates x with no other edit.
• [ ] The check is green. If a file that was already gated goes red only because of the new order, quarantine it in EXCLUDED under a new issue of its own and say so in the pull. Do not abandon the swap for it.

Risk: the swap reshuffles shards, which is what sank #2053 and #2060; that is why this waits on the order fix.