Implement RFC #957: Render Aware Scheduler Interface

[NullVoxPopuli-ai-agent]

Implements the additive API surface of RFC #957 — Render Aware Scheduler Interface (0957-modernized-scheduler).

What this adds

@ember/scheduler

• render(), layout(), composite(), next(), idle() — one function per phase from the RFC, each returning a promise that resolves according to the registered scheduling strategy:

  import { render, layout, composite } from '@ember/scheduler';
  
  async function repositionTooltip(tooltip) {
    await render();    // updated DOM exists, before paint
    await layout();    // read DOM, before paint
    let rect = tooltip.target.getBoundingClientRect();
    await composite(); // write DOM, before paint
    tooltip.element.style.transform = `translate(${rect.x}px, ${rect.y}px)`;
  }

• registerStrategy(strategy) — registers the strategy (an implementation of the RFC's Strategy interface) that the phase functions delegate to, intended to be called once when defining the Application. Per the RFC, the strategy holds no callbacks — it only decides when each phase's promise resolves — so scheduled work keeps its async stack traces and cancellation is handled at the call site.

@ember/scheduler/strategy

The default strategy. It models work as belonging to a Frame and flushes the render → layout → composite windows in order within a single frame, before the next paint, by registering ordered requestAnimationFrame callbacks (microtasks — and therefore work awaiting a phase — flush between them). Semantics follow the RFC's examples:

• scheduling into a phase the current frame hasn't reached yet resolves just-in-time within the current frame
• scheduling into render while the render window is flushing resolves within the current window (recursive render)
• scheduling into a phase the current frame has already flushed (or into layout/composite during their own window) resolves in the next frame
• next() resolves in a task after the current frame completes; idle() uses requestIdleCallback where available
• in environments without requestAnimationFrame (SSR/FastBoot), phases degrade to FIFO timers

What this intentionally does not do (yet)

The RFC's migration story — deprecating @ember/runloop/RSVP, the use-async-scheduler optional feature flag, mapping schedule('render'/'afterRender') onto the new interface, and evolving the reactivity system to schedule renders — is left to follow-up PRs. This PR establishes the public interface and default strategy so that exploration can begin, matching the RFC's intent that strategies be swappable.

Testing

• browser tests for the phase-delegation/registration API and for the default strategy's frame semantics (packages/@ember/scheduler/tests)
• type tests (type-tests/@ember/scheduler-test.ts)
• YUIDoc for all new public API, wired into tests/docs/expected.cjs

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