Add parametric 3D-printable enclosure (OpenSCAD)

A printable reimagining of the wooden Kidsbox: rectangular cabinet with two
front speakers, removable top lid (NFC tag rests there, reader mounts beneath),
and swappable speaker mounts.

- kidsbox.scad: master model + part dispatcher (body/lid/speaker_adapter/grill)
- params.scad: all tunable dimensions in one place
- lib.scad: helper modules (rounded shell, bosses, bolt circles)
- Fixed front-panel speaker interface + swappable adapter so any driver fits
  by printing a new adapter (edit spk_* params, re-export)
- Internal mounts: 2x18650 cradle, ESP32-LyraT standoffs, RC522 under lid,
  3 buttons on lid, USB/charge + power-switch side ports, back-wall vents
- Snap-in speaker grills
- Makefile to export STLs and render previews (headless via xvfb)
- Exported STLs and preview renders included
- Link enclosure from main README

Dimensions are sensible defaults from the reference photos and common parts;
README flags the fit-critical values to verify before mass-printing.

https://claude.ai/code/session_01MhAnh68DPNnTjcKqjNYDmk

README.md

@@ -63,6 +63,17 @@ Once connected to WiFi, open `http://<device-ip>` in a browser.
 
 ---
 
+## Enclosure
+
+A parametric, fully 3D-printable enclosure lives in [`enclosure/`](enclosure/) —
+a rectangular cabinet with two front speakers and a removable top where the NFC tag
+rests, with **swappable speaker mounts** so you can print an adapter to fit your
+drivers. See [`enclosure/README.md`](enclosure/README.md).
+
+![Enclosure](enclosure/img/hero.png)
+
+---
+
 ## Hardware
 
 ### Parts