v0.3.1 — a relation drag that survives the rail taking focus

What was wrong

A relation drag could be silently dropped. On a busy machine, the rail re-renders
the Annotation card shortly after you select a target, and that render moves focus
into the card's note field. The artifact frame lost focus, and the layer read that
as the operator leaving the gesture, so it cancelled the drag it had just shown a
sentence for. You saw "Member 1 should come after Member 2" and nothing was
recorded.

What changed

The drag now survives focus moving anywhere inside the page. It is cancelled only
when the page itself loses focus or goes hidden, which is when the gesture really
can no longer be delivered.

Three browser drives that raced their own setup are now deterministic, and a browser
test that fails on CI keeps a Playwright trace — DOM snapshots, network log and
screencast — so the next unexplained failure carries its own evidence.

No interface, envelope or CLI change. Nothing to migrate.

Full changelog: v0.3.0...v0.3.1