feature: add boardgen command for generating parts files.

This adds a Go utility that generates SVG and JSON files for
adding a new board to the TinyGo Playground simulator parts library.

Signed-off-by: deadprogram <ron@hybridgroup.com>

Author: deadprogram

cmd/boardgen/README.md

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+# boardgen
+
+`boardgen` is a Go utility that generates SVG and JSON files for adding a new
+board to the TinyGo Playground simulator [parts library](../../parts/).
+
+The generated files follow the style conventions documented in
+[parts/README.md](../../parts/README.md): hand-written SVG style, millimeter
+units (1px == 1mm), absolute SVG path commands, and consistent pin/LED shapes
+across boards.
+
+## Usage
+
+```bash
+go run ./cmd/boardgen [flags]
+```
+
+### Examples
+
+**Pico-like board** (horizontal, 20 pins top/bottom, USB on the left, built-in LED):
+
+```bash
+go run ./cmd/boardgen \
+  -name my-pico \
+  -human "My Pico Board" \
+  -pins-top 20 -pins-bottom 20 \
+  -usb left -led \
+  -output ./parts
+```
+
+**Vertical board** (15 pins on each side, USB on top):
+
+```bash
+go run ./cmd/boardgen \
+  -name my-nano \
+  -pins-left 15 -pins-right 15 \
+  -usb top -led \
+  -orientation vertical \
+  -output ./parts
+```
+
+**Four-sided board** (pins on all edges, custom PCB color):
+
+```bash
+go run ./cmd/boardgen \
+  -name my-devkit \
+  -pins-top 10 -pins-bottom 10 \
+  -pins-left 8 -pins-right 8 \
+  -usb left -led \
+  -color "#10A2A1" \
+  -output ./parts
+```
+
+**Minimal board** (only bottom pins, no USB, no LED):
+
+```bash
+go run ./cmd/boardgen \
+  -name my-breakout \
+  -pins-bottom 8 \
+  -output ./parts
+```
+
+**Fixed-size board** (explicit dimensions, rectangular GND pads):
+
+```bash
+go run ./cmd/boardgen \
+  -name my-wide-board \
+  -pins-top 10 -pins-bottom 10 \
+  -width 60 -height 30 \
+  -rect-gnd \
+  -usb left -led \
+  -output ./parts
+```
+
+## Flags
+
+| Flag | Description | Default |
+|------|-------------|---------|
+| `-name` | Board name — used for filenames (`<name>.svg`, `<name>.json`) | *required* |
+| `-human` | Human-readable board name shown in the UI | title-cased `-name` |
+| `-orientation` | Board orientation: `horizontal` or `vertical` | `horizontal` |
+| `-pins-top` | Number of pins on the top edge | `0` |
+| `-pins-bottom` | Number of pins on the bottom edge | `0` |
+| `-pins-left` | Number of pins on the left edge | `0` |
+| `-pins-right` | Number of pins on the right edge | `0` |
+| `-usb` | Side for the USB port: `left`, `right`, `top`, or `bottom` | *(none)* |
+| `-led` | Include a built-in LED (placed next to the USB port) | `false` |
+| `-color` | PCB color as a hex string | `#006837` |
+| `-width` | Board width in mm (0 = auto-sized from pins) | `0` |
+| `-height` | Board height in mm (0 = auto-sized from pins) | `0` |
+| `-rect-gnd` | Use rectangular pad shape for GND pins (taller, rounded corners) | `false` |
+| `-format` | Firmware format string (e.g. `uf2`, `hex`) | `uf2` |
+| `-output` | Output directory for generated files | `.` |
+
+## What it generates
+
+### SVG file (`<name>.svg`)
+
+A board image that matches the project's visual conventions:
+
+- **Millimeter units** — `viewBox` matches the physical board size so that
+  1px == 1mm.
+- **Castellated pads** — half-circle pad shapes matching the Raspberry Pi Pico
+  style. When `-rect-gnd` is set, GND pads use a distinct taller rectangular
+  shape with small rounded corners; otherwise all pads share the same
+  half-circle shape.
+- **Pin attributes** — each pad has `data-pin` (and `data-title` where the
+  display name differs) for the simulator to connect wires.
+- **USB port** — a `#ccc` rectangle protruding 1.3mm past the board edge on the
+  specified side.
+- **LED** — an animated `data-part="led"` element using CSS custom properties
+  (`--color`, `--shadow`) for the simulator to control brightness.
+
+### JSON file (`<name>.json`)
+
+A complete board configuration for the simulator:
+
+- **MCU part** with `GPIO0`–`GPIOn` pin mapping.
+- **LED part** (when `-led` is set) with color and current values.
+- **Wire table** connecting SVG `data-pin` names to MCU GPIOs, power (`vcc`),
+  and ground (`gnd`).
+- **`firmwareFormat`** and **`baseCurrent`** fields.
+
+## Pin assignment
+
+Pins are assigned automatically:
+
+1. **GPIO pins** are numbered sequentially starting from `GP0`, distributed
+   across sides in reading order: top → right → bottom → left.
+2. **GND pins** are auto-inserted after every 5 GPIO pins on each side.
+3. **Power pins** — when `-usb` is specified, the first three positions on the
+   USB side are reserved for `VBUS`, `GND`, and `3V3`.
+4. The **LED** (when enabled) is wired to the last GPIO pin.
+
+## Board sizing
+
+Board dimensions are computed automatically from the pin counts:
+
+- Each pin occupies one **pitch** unit (2.54mm).
+- Pad-depth zones (3.22mm) are added at edges that have pins.
+- Corner gaps prevent overlapping when perpendicular sides both have pins.
+- A minimum board dimension of 12mm is enforced.
+- When USB is present, the `viewBox` extends to include the protruding port.
+
+You can override the auto-computed size with `-width` and/or `-height` (in mm).
+This is useful when you want a specific physical board size, or need extra space
+between pin rows. The pin rows are still positioned based on the computed
+pad-depth offsets, so the board will simply be larger in the overridden
+dimension(s).

cmd/boardgen/go.mod

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+module github.com/tinygo-org/playground/cmd/boardgen
+
+go 1.21