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)