test(react): unit-test MHD/.raw pairing; drop redundant drop-reversed e2e

[korbinian90]

Next step in the unit-first migration (after #247, merged).

Heads-up on the originally-proposed candidate

I offered "DICOM magic-byte sniffing" next, but it turns out isDicomData / isImageType are already thoroughly unit-tested in utility.test.ts (DICM-at-offset-128, all input shapes, wrong-offset, too-short, non-binary). The prior author already did that one. So I pivoted to the adjacent pure logic that was untested: MHD .raw pairing.

What this does

buildImageMessageBodies (utility.ts) is the pairing brain behind the probe-mhd drop specs - it parses each .mhd header's ElementDataFile, finds the sibling .raw (order-independent), attaches it as pairedData, removes the .raw as a standalone image, and emits a loadError when the pair is missing. It was exported but had zero unit coverage - exercised only through real WebGL drop e2e.

• Add buildImageMessageBodies unit tests (in utility.test.ts): order-independent pairing (forward + reversed), case-insensitive basename match, quoted ElementDataFile, backslash paths, ElementDataFile = LOCAL (attached, no pairing), the missing-.raw loadError message, shared-.raw dedup, and non-MHD passthrough. No WebGL - jsdom File-like doubles (buildImageMessageBodies reads only name + arrayBuffer()).
• Remove probe-mhd-drop-reversed.spec.ts: its only unique claim - pairing is independent of drop order - is now a unit test. probe-mhd-drop.spec.ts stays as the real decode-reaches-canvas smoke for the drop path.

Kept in e2e (genuine integration, not touched)

The other six probe-mhd specs each exercise a distinct real decode path - drop (forward), urlparam (auto-fetch paired raw), binary, streamlit-shape, url, and unpaired (the missing-pair warning surfacing on-canvas). The pairing logic is now unit-tested; these keep proving the integration.

Verification

• @niivue/react: 132 unit tests pass (10 files; +9 here).
• turbo lint type-check: green.

🤖 Generated with Claude Code

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Coverage Report

Overall line coverage: 41.9% (+1.1) vs main

Package	Statements	Branches	Functions	Lines
Shared core (packages/niivue-react)	45.7% (+1.6)	46.8% (+1)	44.8% (+0.1)	46.2% (+1.5)
apps/pwa	28.9%	33.3%	52.9%	30.2%
apps/jupyter	14.4%	15.9%	14.9%	14.5%
apps/streamlit	17.8%	5.3%	18.5%	17.7%
apps/vscode	38.7%	39.6%	18.9%	38.1%
apps/desktop-tauri	81.8%	59.1%	78.9%	84.3%

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