Svelte Guideline: Reactivity — Trigger reactivity with assignment. Description: Svelte tracks assignments not mutations. Do: Reassign arrays/objects to trigger update. Don't: Mutate without reassignment. Good Example: items = [...items, newItem]. Bad Example: items.push(newItem). Severity: High. Docs: https://svelte.dev/docs/svelte-components#script-2-assignments-are-reactive.