Shopping Cart State Manager

Problem/Feature Description

A small e-commerce startup is building a product browsing page with a shopping cart sidebar. The engineering team needs a reusable Svelte 5 component that manages cart state: items can be added, removed, or have their quantities updated. The component must display a running total, the count of unique items, and flag when the cart exceeds a configurable maximum item count.

The team recently migrated their codebase from Svelte 4 and wants the new component written entirely using Svelte 5 conventions. The component will consume a static list of available products passed in from the outside, and expose a callback so the parent can react when the user checks out.

A large product catalog (the full inventory JSON, potentially thousands of entries) is sometimes passed in for search/filter purposes, but it is always replaced as a whole rather than mutated item-by-item.

Output Specification

Produce a single file: CartManager.svelte

The component should:

• Accept a list of available products and a max-items limit as inputs from the parent
• Accept a checkout callback so the parent is notified when the user clicks "Checkout"
• Maintain reactive cart state (items and quantities)
• Expose computed summaries: total price, unique item count, and whether the cart is over the limit
• Provide add/remove/update-quantity actions
• Render a minimal UI showing the cart contents, totals, and a Checkout button

Also produce a notes.md file briefly explaining the reactivity approach and which rune was chosen for each piece of state and why.