ci: let every platform report instead of cancelling at the first failure

The test matrix ran with GitHub's default fail-fast, so one red platform
cancelled the other two and the run reported only that something broke
somewhere.

That is not hypothetical here. Seven defects were found in this suite over a
week, and they surfaced one per push rather than together: macOS failing
cancelled Linux, so the Linux failures stayed invisible until macOS was
fixed, and the next Linux failure stayed invisible until that one was. Seven
round trips for what one run could have shown.

It compounded a second problem. Three of the seven could only be reproduced
on a platform not available locally — two needed a machine with no GPU, one
needed a Mac — so CI was the only instrument that could see them, and it was
reporting a third of what it knew.

A cancelled job is not a passing job. A matrix that stops at the first
failure is a one-platform check whose platform is whichever finished first.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>