fix: complete 3D models for STEP export, add board STEP/STL/GLB exports

[thePunderWoman]

Summary

The board's STEP export was missing the ESP32 module, USB-C port, the six side buttons, the RGB LED, the accelerometer, the charger IC, and the DNP thumbstick and XBee sockets. This PR adds working models for all of them, plus a check so it can't silently regress, and adds exports of the assembled board for designing the enclosure.

Why they were missing

• KiCad 10's stock USB_C_Receptacle_HRO_TYPE-C-31-M-12 and SW_Push_1P1T-MP_NO_Horizontal_Alps_SKRTLAE010 footprints point at model files that KiCad 10 doesn't install. They aren't in upstream kicad-packages3D either.
• The ESP32 footprint pointed at an old ${KICAD8_3RD_PARTY} path.
• The custom footprints (SK6812MINI-E-012, QMA6100P, GuliKit_HallStick, BQ25185DLHR, XBee) had no model at all. The BQ25185 had a model in the repo that nothing referenced.

Changes

• PCB/libraries/3dmodels/: models downloaded from EasyEDA/LCSC for the USB-C (C165948), SKRTLAE010 (C110293), QMA6100P (C2887190) and ALPS RKJXV1224005 (C146170, standing in for the GuliKit stick), plus a generated XBee 3 TH outline model. The README records where each came from and how it was aligned.
• scripts/set_3d_models.py: sets the model paths, offsets and rotations in the board and the project footprint libraries. --check fails on any missing or unresolvable model: 13 problems before this PR, 0 after.
• scripts/build_placeholder_3d_models.py: builds the XBee 3 TH model from Digi's mechanical drawing (HRM 90001543).
• Board changes are limited to (model ...) blocks. Pads, copper and footprint positions are untouched.
• Exports in PCB/: snips_controller.step (for modelling), plus snips_controller_color.step and snips_controller.glb (blue solder mask, silkscreen and copper, for viewing), and snips_controller.stl.

How it was checked

• Each model's offset and rotation came from matching its pins, pegs and legs to the footprint's pads. Each part was then exported on its own with kicad-cli and its bounding box compared with its pads in board coordinates. All 13 designators are present in the full export.
• Checked visually with kicad-cli pcb render, top and bottom views.

Caveats / follow-ups

• SK6812MINI-E cutout (possible board bug, needs a look): this is a reverse-mount LED whose body sits 0.84 mm below its pads. KiCad's stock footprint includes a board cutout for it, but SK6812MINI-E-012 has none, so in 3D the LED goes into the board. Not confirmed against the manufacturer's datasheet: the PDF in PCB/datasheets/ is actually for the 5050 SK6812. Worth confirming before the rev2 layout is final.
• The thumbstick model is the ALPS equivalent, so the real GuliKit body may differ. The XBee model is only an outline.
• The USB-C and SKRTLAE010 fixes live only on the board, because those footprints come from KiCad's library. Updating them from the library in KiCad would undo the fix; rerun scripts/set_3d_models.py if that happens.
• The stackup's named solder mask colour "Blue" gets no colour in KiCad's STEP export. The coloured exports were made from a copy using #0B2F8CD4; the board file itself was not changed.
• EasyEDA models aren't clearly licensed for redistribution. Worth considering for a public repo.

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