test: CodSpeed null-change control

[wayfarer3130]

Purpose

This pull request is a control experiment. Do not merge this pull request.

The CodSpeed reports on cornerstonejs/codecs flag benchmarks that the diff cannot
reach. This pull request tests one cause of those reports. The diff changes one
documentation line, and the diff changes no runtime code. A correct report must show
0 regressed benchmarks and 0 improved benchmarks.

The diff

The diff adds one HTML comment line to packages/dicom-codec/README.md. The file is
documentation only. The path is under packages/, so the gate in
.github/workflows/bench.yml benches dicom-codec. The path is not in the
toolchain_touched list and not in the ci_touched list, so the gate does not
force a full sweep.

Preparation

main had no CodSpeed baseline before this test. The tip of main is 841110c,
a chore(release): publish commit, and both workflows skip that commit by design.
The parent commit 1859ac9 also has no baseline, because three build (...) jobs
reached the 50 minute timeout in .github/workflows/pr-checks.yml.

Two manual runs re-seeded the baseline on 841110c:

1. pr-checks.yml, run 34506668677. All 13 jobs report success.
2. bench.yml, run 34506961921. The codspeed-bench job reports success.

Expected result

The CodSpeed comment must show 0 regressed benchmarks, 0 improved benchmarks, and
no unexpected-base footnote. A flagged benchmark shows that a second cause remains.

🤖 Generated with Claude Code

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[codspeed-hq]

Merging this PR will improve performance by 9.37%

⚡ 1 improved benchmark
✅ 16 untouched benchmarks
⏩ 116 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	HTJ2K Lossless (.201)	16 ms	14.6 ms	+9.37%

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing test/codspeed-null-change (64fa512) with main (841110c)}

Footnotes

1. 116 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩