fix: make hydration work correctly with FAST-HTML example

[mohamedmansour]

Two issues prevented the todo-fast example from working after hydration:

1. Class field initializers overwrote hydration state

RenderableFASTElement calls prepare() synchronously during super(). Esbuild compiles @observable items: TodoItemData[] = [] as this.items = [] in the subclass constructor AFTER super() returns, silently overwriting the 3 items that prepare() extracted from the SSR shadow DOM. Fixed by removing class field initializers from @observable properties set in prepare().

2. ObserverMap deep proxies silently swallowed immutable-style updates

With observerMap: 'all', each array item is wrapped with deep observable proxies. When onToggleItem used .map() to create new plain objects, instanceResolverChanged triggered deepMerge which compared proxied getter values via deepEqual and concluded nothing changed, silently dropping the state update. Fixed by mutating items in-place (item.state = 'done') so the observable setter fires directly.

Other fixes:

• Move todo-item loop from light DOM into shadow DOM template and query via this.shadowRoot instead of this
• Always assign this.items in prepare() to guarantee a default
• Add return true to onAddKeydown so the event propagates

[akroshg]

LGTM