docs(shell): size the rear Ethernet, DC and power-button openings

[HuggeK]

The shell page names the three rear openings but never gives their dimensions, so there is no way to tell which coupler or jack fits without opening the STL and measuring it. This adds the numbers.

Measured from the release STLs; identical on Shell-x1202, Shell-x1206 and Shell-x12-a1:

Opening	Size	Centre (model X)
Ethernet	16.0 × 14.0 mm cut-out, plus two Ø4.0 mm screw holes 27.0 mm apart	40.0 mm
DC power	Ø12.5 mm	73.0 mm
Power button	Ø12.5 mm	161.8 mm

The face all three sit in is 3.0 mm thick — the number most often missed, since many panel connectors are cut for thin sheet metal and will not close on 3 mm of plastic.

⚠️ The reference Ethernet coupler does not fit a v1 shell

reference-models/connectors/Ethernet_Panel_Coupler.step is a round coupler with a 21.7 mm thread, and the v1 shells have no round Ethernet bore at all. Sweeping every cut-out in the sloped rear face returns exactly five — the rectangle, its two screw holes, and the two Ø12.5 mm round holes. Nothing at Ø21.7.

That fits how those models arrived (#8 contributed them as references for other enclosure projects), but anyone buying from them for a v1 shell ends up with a coupler that cannot fit. The page now says so. The DC reference jack is the opposite case and is noted as representative: Ø11.0 mm barrel, Ø15.5 mm flange, a correct if slightly loose fit.

Polarity warning

A 5.5 × 2.1 mm barrel plug has no keying, so a centre-negative supply mates perfectly with a centre-positive jack and reverses the rail into the Pi, the UPS and both radars at once. The X1202 and X1206 are both centre pin positive — per @JedS on openflight#273, cited because Geekworm's own wiki states it nowhere.

How the dimensions were measured

The openings lie in one sloped rear face, plane 0.85y − 0.52z = 144.3 (inner) to 147.3 (outer), giving the 3.0 mm thickness. Bore-wall triangles inside that slab were clustered by shared vertices and each cluster measured in the face's own in-plane frame; listing every cluster with no size filter is what rules out a missed opening.

The Ethernet cut-out resolves to exactly 8 triangles, confirming sharp corners rather than rounded, at 16.00 × 14.00 mm. The round holes come out at Ø12.50 (12.47–12.53 across the facets) and the screw holes at Ø4.00, all through the 3.01 mm wall. Re-running on all three v1 shells returns identical numbers.

These are measured from the meshes rather than read off the CAD source, so please sanity-check them against the Fusion model before merging — particularly the 27.0 mm screw spacing, since that is the one a buyer cannot fudge.

🤖 Generated with Claude Code

[HuggeK]

Read through it - looks fine to me.