CI structure amplifies flakes: fail-fast matrix + test-complete mirror check

[schani]

What happens

Two aspects of the fixture-test workflow (.github/workflows/test-pr.yaml) inflate the blast radius of every flaky job and hide the results of unrelated languages.

1. fail-fast: true on the fixture matrix. The test job's matrix (strategy.fail-fast: true, line 34) runs every language fixture as a matrix leg. When any one leg fails — including a pure infra flake — GitHub cancels all sibling legs. A single flaky job therefore turns into a wall of "cancelled" jobs, and we lose the signal of whether the other languages would have passed.

2. test-complete only mirrors the matrix. The test-complete job (line 211) runs if: cancelled() || contains(needs.*.result, 'cancelled') || contains(needs.*.result, 'failure') and simply prints "Some workflows have failed!" and exit 1. It never fails independently — it only reflects whatever the matrix did. During triage its failure carries no information beyond "some matrix leg failed or was cancelled."

Evidence

• .github/workflows/test-pr.yaml line 34: fail-fast: true on the test matrix.
• .github/workflows/test-pr.yaml lines 211-218: test-complete gate that mirrors the matrix result.
• Today's triage of 31 failing PRs: several PRs showed many "cancelled" sibling jobs stemming from a single flaky leg, and test-complete "Some workflows have failed!" was never an independent failure — it only mirrored whichever matrix job failed.

Impact

• One flaky job (Scala Bloop timeout, Elm warmup deadlock, cjson download reset) cancels the entire language matrix.
• Triage is harder: the real failing leg is buried among cancellations, and passing languages never report.
• The test-complete red X is noise on its own; you must open the matrix to learn anything.

Suggested solutions

• Set fail-fast: false on the fixture matrix so each language leg runs to completion independently and one flake stops masking the rest.
• During triage, treat test-complete as a mirror only — inspect the individual matrix legs, not the aggregate gate.

Related flaky areas

These are the specific flakes this structure amplifies:

• scala3-upickle CI flake: Bloop compiler-server startup timeout + maven-nightlies download failures #3063 — scala3-upickle: Bloop startup timeout + maven-nightlies download failures
• elm CI flake: Warmup.elm fixture setup races the compiler (elm-stuff file locks / MVar deadlock) #3064 — elm: Warmup.elm setup races the compiler (file locks / MVar deadlock)
• cjson CI flake: cJSON.c downloaded from raw.githubusercontent.com at test time #3065 — cjson: cJSON.c downloaded from raw.githubusercontent.com at test time

Interim policy

Per CLAUDE.md ("Known CI flakiness"), we accept these flakes for now and retry the failed jobs (gh run rerun <run-id> --failed) when they occur.