Build guide: parts, printing, assembly in order, wiring, first run

BUILD.md walks through the whole build: what to buy (parts and screws), the
v3 print list with materials and supports, case, electronics (microstep
jumpers, driver current), centre, the arm on the bench, hub and slip ring,
wiring to the CNC shield, software setup and a first run without sand, then
floor, magnet height, sand and glass, plus troubleshooting.

Also adds the 608 fit test to print/v3 so the v3 folder is complete.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

print/README.md

@@ -18,7 +18,8 @@
 ## Full parts list (v3)
 | Part | Folder | Notes |
 |---|---|---|
-| 00b fit test 6804 | `v3` | **Print first.** Checks the new bearing fits (below). |
+| 00 fit test 608 | `v3` | **Print first.** Pins and pockets for the 608 bearings (carriage rollers, idler). The parts use the 8.1 mm pin and 22.2 mm pocket. |
+| 00b fit test 6804 | `v3` | **Print first.** Checks the hub bearing fits (below). |
 | 01–04 shell Q1–Q4 | `v3` | Upright, as before. The deck bosses have a 45° underside, so no supports. |
 | 05–08 deck Q1–Q4 | `v3` | Upside down, as before. Deck Q1 has the correct Uno standoffs, so `28` isn't needed. |
 | 09 center plate | `v3` | Upside down, as before. |