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

docs/DESIGN.md

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+# SN-L00 Design Document
+
+## Overview
+
+The SN-L00 is a 6 HP Eurorack module that measures round-trip latency in audio/CV signal chains.
+
+## Block Diagram
+
+```
++12V ──┬── LDO ──── 3.3V ──── RP2040-Zero ──── OLED Display
+       │                          │    │
+       │                          │    └── I2C (SDA/SCL)
+       │                          │
+GND ───┴──────────────────────────┤
+                                  │
+              ┌───────────────────┼───────────────────┐
+              │                   │                   │
+         TRIG OUT            RETURN IN            BUTTON
+              │                   │                   │
+         74LVC1G17            MCP6001            Tactile
+         (buffer)            (buffer)             switch
+              │                   │
+              ▼                   ▼
+         3.5mm Jack          3.5mm Jack
+```
+
+## Signal Flow
+
+### Trigger Output
+1. RP2040 GPIO generates 5ms pulse (configurable)
+2. 74LVC1G17 Schmitt buffer cleans edge, provides drive strength
+3. Output: 0-3.3V trigger compatible with Eurorack gate inputs
+
+### Return Input
+1. Eurorack signal (0-5V typical, can be ±5V audio)
+2. Protection diodes clamp to 3.3V rail
+3. MCP6001 op-amp buffers signal
+4. Voltage divider if needed for >3.3V signals
+5. RP2040 GPIO reads with internal Schmitt trigger
+
+### Measurement
+1. RP2040 starts timer on trigger output rising edge
+2. Timer stops on return input rising edge
+3. Latency = timer count × timer resolution
+4. Display updates with result in ms
+
+## Specifications
+
+| Parameter | Value |
+|-----------|-------|
+| Width | 6 HP (30.00mm) |
+| Depth | <25mm (skiff-friendly) |
+| Power +12V | ~20mA |
+| Power -12V | 0mA (not used) |
+| Trigger output | 0-3.3V, 5ms default |
+| Input threshold | ~1.5V (adjustable in firmware) |
+| Resolution | <10μs (RP2040 @ 125MHz) |
+| Range | 0.01ms - 999ms |
+| Display | 128x32 OLED, I2C |
+
+## Operating Modes
+
+### Mode 1: Single Shot
+- Press button to trigger measurement
+- Display shows result until next measurement
+- LED blinks on trigger
+
+### Mode 2: Continuous
+- Hold button 2s to enter continuous mode
+- Triggers every 500ms, displays live reading
+- Shows min/max/average
+- Press button to exit
+
+### Mode 3: Statistics
+- After measurements, short press cycles through:
+  - Last reading
+  - Minimum
+  - Maximum
+  - Average (last 10)
+
+## Panel Layout (6 HP)
+
+```
+┌─────────────────────┐
+│      SN-L00         │
+│                     │
+│   ┌───────────┐     │
+│   │   OLED    │     │
+│   │  12.4 ms  │     │
+│   └───────────┘     │
+│                     │
+│       [BTN]         │
+│                     │
+│   ○ OUT    ○ IN     │
+│   TRIG    RETURN    │
+│                     │
+│        ● PWR        │
+└─────────────────────┘
+```
+
+## Firmware Architecture
+
+```
+main.c
+├── init_gpio()          # Configure pins
+├── init_i2c()           # OLED communication
+├── init_timer()         # High-resolution timer
+│
+├── trigger_pulse()      # Generate output pulse
+├── wait_for_return()    # Monitor input with timeout
+├── calculate_latency()  # Timer math
+│
+├── display_update()     # OLED rendering
+├── button_handler()     # Mode switching
+└── main_loop()          # State machine
+```
+
+## GPIO Assignments (RP2040-Zero)
+
+| GPIO | Function | Notes |
+|------|----------|-------|
+| GP0 | I2C SDA | OLED |
+| GP1 | I2C SCL | OLED |
+| GP2 | Trigger Out | Via 74LVC1G17 |
+| GP3 | Return In | Via MCP6001 |
+| GP4 | Button | Internal pullup |
+| GP5 | Power LED | Optional |
+
+## Future Expansion Ideas
+
+- CV output proportional to latency
+- MIDI output (latency as CC)
+- USB logging to computer
+- Multiple trigger patterns (burst, random)
+- Frequency response mode (sweep test)

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*