Cancel superseded CI runs so Pages deploys stay in order

[yyolk]

Why

Two master CI runs overlapped: older #180 serial pages still running while #181 matrix pages started. Without concurrency, the older deploy can finish last and overwrite the newer catalog.

Change

Add GitHub Actions concurrency to .github/workflows/ci.yml (after on:, before jobs:):

concurrency:
  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
  cancel-in-progress: true

One group per PR number or branch ref; newer runs cancel superseded ones so Pages cannot deploy out of order.

publish.yml and release-pdfs.yml are unchanged. No job logic changes.

  

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