Improve Consistency for Lazy Lifecycle (watched) across API

[estherbrunner]

0.18.1

Added

• Memo watched(invalidate) option: createMemo(fn, { watched }) accepts a lazy lifecycle callback that receives an invalidate function. Calling invalidate() marks the memo dirty and triggers re-evaluation. The callback is invoked on first sink attachment and cleaned up when the last sink detaches. This enables patterns like DOM observation where a memo re-derives its value in response to external events (e.g., MutationObserver) without needing a separate Sensor.
• Task watched(invalidate) option: Same pattern as Memo. Calling invalidate() aborts any in-flight computation and triggers re-execution.
• CollectionChanges<T> type: New typed interface for collection mutations with add?: T[], change?: T[], remove?: T[] arrays. Replaces the untyped DiffResult records previously used by CollectionCallback.
• SensorOptions<T> type: Dedicated options type for createSensor, extending SignalOptions<T> with optional value.
• CollectionChanges export from public API (index.ts).
• SensorOptions export from public API (index.ts).

Changed

• createSensor parameter renamed: start → watched for consistency with Store/List lifecycle terminology.
• createSensor options type: ComputedOptions<T> → SensorOptions<T>. This decouples Sensor options from ComputedOptions, which now carries the watched(invalidate) field for Memo/Task.
• createCollection parameter renamed: start → watched for consistency.
• CollectionCallback is now generic: CollectionCallback → CollectionCallback<T>. The applyChanges parameter accepts CollectionChanges<T> instead of DiffResult.
• CollectionOptions.createItem signature: (key: string, value: T) => Signal<T> → (value: T) => Signal<T>. Key generation is now handled internally.
• KeyConfig<T> return type relaxed: Key functions may now return string | undefined. Returning undefined falls back to synthetic key generation.

Removed

• DiffResult removed from public API: No longer re-exported from index.ts. The type remains available from src/nodes/list.ts for internal use but is superseded by CollectionChanges<T> for collection mutations.