Add documentation for float sort_order pattern

Document the float sort_order pattern with TypeScript examples for calculating sort order and reindexing when precision is low.

patterns/sort-order.md

@@ -0,0 +1,29 @@
+# Patroon: Float sort_order (drag-and-drop volgorde)
+
+## Berekening bij tussenvoeging
+
+```ts
+function getSortOrder(before: number | null, after: number | null): number {
+  if (before === null && after === null) return 1.0
+  if (before === null) return after! / 2
+  if (after === null) return before + 1.0
+  return (before + after) / 2
+}
+```
+
+## Herindexeer als precisie opraakt
+
+Trigger wanneer het kleinste verschil tussen twee opeenvolgende items < 0.001 is.
+
+```ts
+async function reindexIfNeeded(items: { id: string; sort_order: number }[]) {
+  const minGap = Math.min(...items.slice(1).map((item, i) =>
+    item.sort_order - items[i].sort_order
+  ))
+  if (minGap < 0.001) {
+    await Promise.all(items.map((item, i) =>
+      prisma.pbi.update({ where: { id: item.id }, data: { sort_order: i + 1.0 } })
+    ))
+  }
+}
+```