EPBDS-16211: every master commit gets a CI run of its own

[AlexSamBY]

What

Each push to master gets a CI run of its own. A push to master now has a concurrency group named by its commit (github.sha). A pull request's group is still its ref, and its newer push still cancels the run before it.

Why #205 was not enough

#205 stopped a push to master from cancelling the run in progress: cancel-in-progress is on for pull requests only. But a group still holds at most one running run and one pending run. GitHub cancels the pending one when another is queued, whatever cancel-in-progress says. From GitHub's documentation: "any existing pending job or workflow in the same concurrency group will be canceled".

It happened on 2026-10-12, when #214, #215 and #216 merged within two minutes:

commit	its run was created	what happened to it
1b16e4d (#214)	16:12:35	ran until 16:18:20, success
5cb6bf6 (#215)	16:13:29	pending behind it; cancelled at 16:14:02 with no job started
1710577 (#216)	16:14:01	success

5cb6bf6 is the case #205 set out to prevent: a master commit with no finished run.

The change

• .github/workflows/ci.yml: the group is ci-${{ github.workflow }}-${{ github.event_name == 'push' && github.sha || github.ref }}. Master's runs no longer share a group, so none waits and none is cancelled; merges in quick succession run in parallel. The comment says why one group for the branch was not enough.
• Why not queue: max: it keeps up to 100 pending runs per group, but GitHub's documentation says it cannot be combined with cancel-in-progress: true, which pull requests use.
• The changelog: its entry said "CI cancels a run only when a newer push to the same pull request supersedes it". It now says that each master commit gets a run of its own.

The same fix for release/v0.34's ci-0.34.yml is #218.

Verified

• The workflow parses (js-yaml).
• For a push, the group comes out as ci-CI-<sha>; for a pull request, as ci-CI-refs/pull/<n>/merge. && and || return one of their operands, the common way to pick a value in a workflow expression. This PR's own run evaluates the expression, so a syntax error would have kept it from starting.
• This PR's own run uses the pull-request group, which is unchanged. The push groups show on the next merges in quick succession: each commit should get a finished run.

Gates

All 22 gates and the manifest contract exit 0 on HEAD, on Node 24.14.0.

• The suite has 2875 tests. All pass on React 18 and 19.
• On React 16.14 and 17, 2871 pass and EPBDS-16211: contracts for a document inside a transition, and for what one edit costs in a 50-row table #207's 4 transition tests are skipped, as before.
• In the browser, the demo's 62 tests pass in Chromium on React 18 and 19, and so do the packed tarball's on React 16.14 to 19.

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