perf(concat): block-copy concatenation instead of SeqRecord round-trip (89x faster, -2.6 GB) - #90

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, codecov/codecov-action@v4, prefix-dev/setup-pixi@v0.8.1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

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