update all docs: Orange Pi 3B 8GB purchased (89)
- docs/sbc-selection.md: full rewrite for OPi 3B specs (RK3566, 8GB, 256GB eMMC, built-in WiFi)
- README.md: SBC description, checklist, components table, quick ref, cost estimate
- docs/parts-list.md: SBC marked purchased, eMMC/controller/WiFi changes
- docs/design-decisions.md: WiFi exclusion reason, native eDP, power input
- docs/salvage-analysis.md: WiFi card not needed, cooling fan, summary tables
- docs/storage-architecture.md: rewritten for 256GB eMMC primary boot
- docs/usb-expansion.md: 2x USB3 native, port allocation, power budget
- docs/port-layout.md: OPi3B port differences (eDP, power-only USB-C, built-in WiFi)
- docs/power-system.md: lower power draw context
BarnacleBoy
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