This PR migrates the core reactive reactivity system in web_core from RxJS to Preact Signals.

Motivation

The migration to Preact Signals significantly simplifies the data-binding layer and aligns with a more modern, synchronous state management paradigm. RxJS Observable streams, while powerful, introduce overhead when handling simple state properties and make synchronous resolution complex (e.g., needing to emit an initial value synchronously upon subscription). Signals inherently hold a current value and provide granular reactivity with less boilerplate.

Key Architectural Changes

1. DataContext Reactivity Model

   • Removed all RxJS dependencies (Observable, combineLatest, switchMap, etc.).
   • Introduced resolveSignal<V>(), which converts A2UI DynamicValue objects (literals, paths, and function calls) into a reactive Preact Signal<V>.
   • subscribeDynamicValue now creates a Preact effect that watches the underlying Signal and emits changes to the subscriber.

2. DataModel Updates

   • The DataModel was refactored to manage a map of paths to Signal instances rather than relying on custom RxJS subjects.
   • Introduced a shallow-cloning mechanism during updates to ensure object/array reference changes correctly trigger Signal updates (as Signals use strict equality).

3. Function Execution & Resource Management (AbortSignal)

   • Crucial Fix: Addressed a fundamental impedance mismatch between Observables and Signals. Observables couple data emission with lifecycle teardown (via subscription disposal). Signals do not.
   • To prevent resource leaks (like dangling setInterval timers in the metronome function), introduced the Cancellation Token Pattern using standard web AbortSignal APIs.
   • Functions that manage long-running resources now receive an abortSignal?: AbortSignal parameter.
   • DataContext now creates an AbortController for each reactive function execution. It aborts the controller when the function arguments change (triggering a re-evaluation) or when the component utilizing the function is disposed.

4. Basic Functions & Catalog

   • Refactored BASIC_FUNCTIONS (like formatString) to consume and return Signals instead of Observables. formatString now uses computed() to combine dynamic parts.

5. Test Migration

   • Migrated all unit tests across the framework to assert against Signals and synchronous value resolution, replacing RxJS subscription testing patterns.