Master Svelte reactive state patterns

Guide Svelte 5 reactive state, props, and effects via rune selection and migration

Quick rune selection and Svelte 4→5 migration guidance to prevent reactivity bugs

• · component code snippet
• · reactivity question (state/computed/side-effect)

• · correct rune ($state, $derived, $effect, $props, $bindable)
• · working code example

• · Svelte 5.x project
• · no Svelte 4 syntax mixing

• · reactivity missing if rune placed in function instead of top-level
• · overridden $derived uses wrong syntax

• · distinguishes top-level vs scoped
• · warns against 4/5 mixing
• · references migration-gotchas.md