feat: scaffold Svelte app with shared types and scale calculation

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/App.svelte

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+<script lang="ts"></script>
+
+<h1>Scrap Tracer</h1>

src/lib/scale.ts

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+import type { Point } from './types';
+
+export function scaleFromLine(a: Point, b: Point, lengthMm: number): number | null {
+  const lengthPx = Math.hypot(b.x - a.x, b.y - a.y);
+  if (!(lengthMm > 0) || lengthPx < 1) return null;
+  return lengthPx / lengthMm;
+}

src/lib/types.ts

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+export type Point = { x: number; y: number };
+
+/** A piece outline: one outer ring plus hole rings. Units depend on the pipeline stage (px or mm). */
+export type Shape = { outer: Point[]; holes: Point[][] };
+
+/** RGBA pixels, structurally compatible with ImageData. */
+export type RasterImage = { width: number; height: number; data: Uint8ClampedArray<ArrayBuffer> };
+
+export type DetectParams = {
+  threshold: number;
+  invert: boolean;
+  seed: Point | null;
+  /** Negative shrinks the piece (and grows holes). */
+  marginPx: number;
+};

src/main.ts

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+import { mount } from 'svelte';
+import App from './App.svelte';
+
+mount(App, { target: document.getElementById('app')! });