feat(ark-flow): add Rev 2 case in STEP with board-slot clearance fix

[dakejahl]

Summary

Adds the ARK Flow case as STEP (with matching STL) at Rev 2, based on revised files a customer sent us, with one interference defect corrected before publishing. Rev 1 STLs are kept until a test print validates Rev 2.

Problem

The case/ directories in this repo only ship STL, which is a triangle mesh — customers can't mate to it, dimension it, or modify it without a mesh-to-solid conversion. That is what prompted the customer to send STEP files back to us.

Comparing his files against Rev 1 showed they are not just a format conversion — the geometry changed. The significant change is the board slot: Rev 1 seats the board on a shelf 0.8000 mm below the parting face against a top-half ledge 0.7218 mm above it, giving a 1.5218 mm slot. That is exactly the modelled PCB thickness of ARK_Flow_Rev_2_3D_Model.step, to four decimals — zero clearance, which fits the CAD model but not a real board with normal thickness tolerance. His revision opens it to 1.0000 + 0.7000 = 1.7000 mm.

The revision also introduced a defect. The connector cut-out back wall in the bottom half moved inward from y=4.8531 to y=4.6190. Rev 1 cleared the bottom-side component at x[10.94, 18.94] by exactly 0.2000 mm; the revision spent that entire margin and went 0.034 mm past it, for 0.092 mm³ of interference. Every other change in the revision moves outward to add clearance, so this reads as an oversight rather than intent.

Solution

Restored that one wall to its Rev 1 position, leaving the rest of the revision intact. The cut removes 1.5735 mm³ and the part remains a single valid solid with an unchanged outer envelope.

Verified with boolean interference using ark-hardware-jigs:

Check	Result
Bottom + top vs ARK_Flow_Rev_2_3D_Model.step, board seated	0.000000 mm³
Bottom vs ARK_Flow_MR_Rev_1.0.step	0.000000 mm³
Top half against bottom half (halves mate)	0.000000 mm³
Board slot vs 1.5218 mm PCB	+0.1782 mm clearance

Both halves are written in the same coordinate frame as ARK_Flow/model/, matching the existing Rev 1 STLs, so the case and PCBA open already assembled.

Two things to confirm before this is merged:

• Revision number. Rev 2 is our label. If the customer's files correspond to an official revision, or if this should instead land upstream in ARK_Flow and be mirrored here per manifest.tsv, the naming and the manifest rows should follow that.
• ARK Flow MR. ARK_Flow_MR/case/ currently holds byte-identical STLs to ARK_Flow/case/. Rev 2 clears the MR board too, but it is not added there in this PR — leaving MR on Rev 1 is an inconsistency worth a decision.

[dakejahl]

@NickJ98 let's print this off and verify it. It would also be good to find the .step files for the other cases as .step is strictly better than .stl