Fix holder geometry export

[dementive]

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[JustinSGray]

Good catch, and it's a real defect — but it can't land here, because the code this edits no longer exists on the branch this would merge into.

efaa214 ("Take the packaged Fusion exporter", open as #9) deleted app/shared/fusion-library.ts and its test and moved the export into @toolpath/tool-support/export/fusion, whose per-type rules come from Autodesk's published JSON Schema. app/shared/fusion-input.ts is the whole seam that remains. Merging this to main and then main into paul/tool_catalog is a modify/delete conflict on both files, and the resolution is to take the deletion:

CONFLICT (modify/delete): apps/catalog/app/shared/fusion-library.ts
  deleted in HEAD and modified in nathan/fix-holder-export

The bug survived the move, so the finding still stands. fusionHolder's published arm does exactly what holderOf did here — writes holder.gaugeLength against a stack that fromPublished builds out to projection. The measured arm was already right: it cuts at the gage line with belowGageLine and takes the gauge length from the last vertex.

Carried upstream as toolpath/ui-packages#114, with the reasoning and the numbers written down:

• gaugeLength is derived from the exported stack on both arms, so a document can never again state one length while drawing another.
• Where the vendor's figure disagrees, a dropped note names both numbers — on a BT 30, REGO-FIX's B4 - B3 is 48.4 mm, the gauge-line-to-flange distance, and that's the whole gap.
• assemblyGaugeLength needed nothing: geometry.ts already takes the holder gauge from the fusionHolder result, so it's null when the holder is dropped. That's your second change, already correct upstream.

Both fixtures published projection === gaugeLength, which is why nothing caught this — here and upstream both. #114 adds a case where they differ; four of its five assertions fail against the old behaviour.

Closing this one as superseded. The template-side follow-up (dep bump, fixture, and the 'gauge length' warning test whose meaning changes) lands after #114 releases and #9 merges.