Join 11: gitea reads the step event's fingerprint by the seat's name

The seat mesh-pipeline-runner's step event carries `fingerprint`; gitea read
`fp`, which no event carries, and which the type-check refuses once the Node
fetch places the runner's contracts (design 47 G A-285).

Builds: design 47 G A-285

Claude-Session: https://claude.ai/code/session_01NcbgfGKaxw984XyAPfLm4h

modules/gitea/test/failedsteps.test.ts

@@ -48,6 +48,9 @@ test("a head is refused by a required step that failed at its fingerprint in any
   await f.stepEnded(ended(C3, "failed", { step: "lint", status: "optional", fingerprint: "fp-2" }));
   await judged("a failed optional step", C3, "run-on-request nats unit");
   assert.equal(kv.has(failedStepKey("nats", "fp-2", "lint")), false);
+  // A result that names no fingerprint is kept at the one the seat's step event carries (`fingerprint`).
+  await f.stepEnded(ended(C3, "failed", { step: "lint", fingerprint: undefined }, { fingerprint: "fp-3" }));
+  assert.ok(kv.has(failedStepKey("nats", "fp-3", "lint")), "the event's fingerprint is kept");
   // A plain pass clears nothing; a rerun that passes clears it.
   await f.stepEnded(ended(C2, "passed"));
   await judged("a plain pass", C2, "run-on-request nats unit");