refactor: Extract Block Grid render-timing decision into a pure, tested module

[rickbutterfield]

Summary

Extracts the Block Grid preview's "should I re-render yet" decision — previously four mutable fields read via inline conditionals at two call sites in block-grid-preview.custom-view.element.ts — into a new, pure, independently unit-tested module (render-scheduler.ts).

• decideGridRenderTrigger / shouldDeferInitialGridRender — pure functions, no DOM, no this, no timers.
• ResizeDebouncer — replaces the inline setTimeout/clearTimeout pair.
• BlockGridPreviewCustomView is wired to call these instead of inlining the logic.

This is the render-timing decision behind issues #293/#294 (Block Grid area previews rendering blank or before layout data was available) — it had never had a dedicated unit test until this branch. A new element-level test (grid-render-timing.test.ts) drives real observable state through the element to prove the wiring reproduces the fix end-to-end, not just that the extracted functions are individually correct.

One subtle, deliberate behavior change surfaced during review: the pure function's single-return-value design makes the "layoutAreas arrived" and "resize" triggers mutually exclusive on the same observable emission, where the old code could fire both (an immediate render plus a redundant 300ms-later one). This is locked down with an explicit test and comment rather than left implicit — the likely effect is one fewer redundant preview request, not a behavior regression.

Scoped to Block Grid only — List/Single/RichText don't have this timing complexity and are untouched. A follow-on plan (collapsing all four preview elements onto shared descriptors) is intentionally out of scope here.

Test plan

• npm test — 32/32 passing
• Two rounds of automated code review (task-level + whole-branch), both approved with fixes applied
• Manual backoffice smoke test (Block Grid with at least one area, resize behavior) — not run this cycle, no browser access available; recommend running before a release tag

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