BUG: Streaming test baselines stale for OpenSlide 3.4.1 (itkOpenSlideTestStreaming, itkOpenSlideTestApproximateStreaming) #40

Millions of differing pixels, mean error > 100 — this is a decode-path difference, not numeric drift. 7 of 9 tests pass, so the module itself is healthy.

• Runner: ubuntu-22.04, system libopenslide-dev 3.4.1+dfsg-5build1.
• main reproduces: latest "Build, test, package" run on main is a failure (2026-04-23); prior main runs in Nov 2025 and Mar 2025 fail the same way.
• First surfaced by PR COMP: Modernize CMake and update CI to v5.4.6 with OpenSlide system packages #37 (COMP: Modernize CMake and update CI to v5.4.6 with OpenSlide system packages, merged 2026-04-03), which is what first let CI actually build and run the test suite against a system OpenSlide.
• Stale baseline: test/Input/CMU-1-level-1-stream-200.mha (and its sibling streaming baseline).

1. Build locally against OpenSlide 3.4.1 and run only the two streaming tests.
2. Capture the new outputs and confirm by eye that they are correct renderings (not a second wrong answer).
3. Upload the regenerated .mha baselines to ExternalData and update the content links.
   Note: this repo migrated from .md5 to .cid ExternalData tags in PR ENH: Convert ExternalData .md5 tags to .cid (IPFS content IDs) #38 (merged 2026-04-22), and PR ENH: Convert ExternalData .md5 tags to .cid (IPFS content IDs) #39 continues that migration and is still open — regenerate after ENH: Convert ExternalData .md5 tags to .cid (IPFS content IDs) #39 lands, and write .cid tags, not .md5.
4. Alternative if the decode path is expected to keep drifting across OpenSlide versions: give these two tests an explicit comparison tolerance / an allowed number of differing pixels rather than pinning a byte-exact baseline to one OpenSlide release.