Add a printable SnapLine desk cable manager

prints/cable-manager/README.md

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+# SnapLine Cable Manager
+
+Desk cable comb for Bambu Studio. Open `cable-manager.3mf` in the **Prepare** tab, then **Slice plate**.
+
+**80.6 × 28.0 × 15.6 mm** · six snap-in slots · prints flat, no supports
+
+## Slots
+
+Cables drop in from the top. The opening is narrower than the jacket, so they click in.
+
+| Label | Opening | Typical cable              |
+| ----- | ------- | -------------------------- |
+| 4     | 4.4 mm  | thin USB-C, Lightning, aux |
+| 5     | 5.4 mm  | USB-C / Lightning jackets  |
+| 6     | 6.4 mm  | USB-A, thicker USB-C       |
+| 7     | 7.6 mm  | HDMI, DisplayPort          |
+| 9     | 9.0 mm  | DC barrel, chunky USB      |
+| 11    | 11.2 mm | laptop power brick         |
+
+Raised numbers sit on the front edge and are a second object in the `.3mf` — assign them a contrast filament on the H2C (white or pink on black).
+
+## Print (H2C)
+
+- Printer: Bambu Lab H2C
+- Plate: Textured PEI
+- Profile: `0.20mm Standard @BBL H2C`
+- Material: PLA (PETG is fine)
+- Supports: off
+- Infill: 15 %
+- Walls: 2 or 3
+- Brim: off
+- Stick-down: double-sided tape on the flat bottom
+
+About 12–18 g of PLA. One colour works; two colours need no AMS if you map body → left nozzle and labels → right nozzle.
+
+## Files
+
+| File                | Use                                   |
+| ------------------- | ------------------------------------- |
+| `cable-manager.3mf` | Open this in Bambu Studio             |
+| `cable-manager.stl` | Geometry only, if you slice elsewhere |
+| `preview.svg`       | Front + top drawing                   |
+| `generate.py`       | Rebuild after changing sizes          |
+
+```bash
+python3 prints/cable-manager/generate.py --check
+```
+
+Slot diameters and the snap ratio live at the top of `generate.py`.