Add BOM and design documentation

- BOM with LCSC part numbers for PCBWay assembly
- Hand-solder module sources (RP2040-Zero, OLED)
- Design doc with block diagram, specs, GPIO pinout
- Panel layout concept for 6 HP

hardware/BOM.md

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+# SN-L00 Bill of Materials
+
+## PCBWay Assembly (LCSC parts)
+
+| Ref | Description | Value | Package | LCSC # | Qty |
+|-----|-------------|-------|---------|--------|-----|
+| C1,C2 | Capacitor, ceramic | 100nF | 0603 | C14663 | 2 |
+| C3,C4 | Capacitor, ceramic | 10uF | 0805 | C15850 | 2 |
+| C5 | Capacitor, electrolytic | 47uF/25V | SMD 6.3x5.4 | C134767 | 1 |
+| R1-R4 | Resistor | 10K | 0603 | C25804 | 4 |
+| R5,R6 | Resistor | 1K | 0603 | C21190 | 2 |
+| R7 | Resistor | 100K | 0603 | C25803 | 1 |
+| D1 | Schottky diode (reverse polarity) | SS14 | SMA | C2480 | 1 |
+| D2 | LED, green (power) | Green | 0603 | C72043 | 1 |
+| U1 | LDO regulator 3.3V | AMS1117-3.3 | SOT-223 | C6186 | 1 |
+| U2 | Schmitt trigger buffer | 74LVC1G17 | SOT-23-5 | C388628 | 1 |
+| U3 | Op-amp (input buffer) | MCP6001 | SOT-23-5 | C93025 | 1 |
+| J1 | Eurorack power header | 2x5 shrouded | 2.54mm | C429954 | 1 |
+| J2,J3 | Thonkiconn jack | PJ398SM | - | C2935955 | 2 |
+| SW1 | Tactile switch | 6x6mm | SMD | C127509 | 1 |
+
+## Hand-Solder Modules
+
+| Ref | Description | Source | Part # | Approx Price | Qty |
+|-----|-------------|--------|--------|--------------|-----|
+| MOD1 | RP2040-Zero | AliExpress / Waveshare | RP2040-Zero | ~€2-3 | 1 |
+| MOD2 | OLED 128x32 I2C | AliExpress | SSD1306 0.91" | ~€2-3 | 1 |
+
+### Alternative sources for modules
+
+**RP2040-Zero:**
+- Waveshare official: https://www.waveshare.com/rp2040-zero.htm (~$4)
+- AliExpress: Search "RP2040-Zero" (~€2-3)
+- Amazon: Search "RP2040-Zero" (~€5-8)
+
+**OLED 128x32 I2C:**
+- AliExpress: Search "0.91 inch OLED I2C SSD1306" (~€2)
+- Amazon: Search "0.91 OLED Arduino" (~€5-7)
+- Adafruit: Product 4440 ($13) - quality option
+
+## Eurorack Jacks (Alternative Sources)
+
+Thonkiconn PJ398SM may have limited LCSC stock. Alternatives:
+- Thonk.co.uk: https://www.thonk.co.uk/shop/thonkiconn/ (€0.40/ea, bulk)
+- Exploding Shed: https://www.exploding-shed.com/
+- Modular Addict: https://modularaddict.com/
+
+## Notes
+
+- All SMD passives are 0603 for easy assembly
+- AMS1117-3.3 handles 12V→3.3V efficiently for low-power MCU
+- 74LVC1G17 provides clean trigger output with Schmitt hysteresis
+- MCP6001 buffers input signal, rail-to-rail on 3.3V
+- Reverse polarity protection via D1
+
+## Estimated Costs
+
+| Category | Cost |
+|----------|------|
+| PCB (5pcs, PCBWay) | ~€5-10 |
+| SMD Assembly | ~€15-25 |
+| LCSC components | ~€3-5 |
+| Hand-solder modules | ~€5-8 |
+| Jacks (if separate) | ~€1-2 |
+| **Total per unit** | **~€25-40** |
+
+*Prices approximate, bulk reduces significantly*