Fix lazy-rendered charts staying blank on script load-order race

[lucadobrescu]

Closes #1319

Root cause

visualizerLoadScripts() fetched all script[data-visualizer-script] placeholders with parallel jQuery.getScript() calls, discarding the WordPress dependency order. When render-facade.js executed before the chart renderer (render-google.js etc.), the facade fired its one-shot visualizer:render:chart:start event with no listener registered. all_charts was never set, so every later resize-based recovery iterated an empty object — the chart stayed blank until a full page reload.

Fix

Replace each placeholder with a real <script> element using async = false: browsers download the scripts in parallel but execute them in insertion order, which is the order WordPress printed them (renderer before facade). Side benefits: no more _=timestamp cache-busting on every page view, and one failed script no longer affects the others.

Reproduction / verification

Deterministic repro: delay render-google.js by a few seconds (slow CDN / cache-miss simulation), load a page with a lazy-render chart, interact. Before: blank forever. After: renders once scripts arrive. Verified with free 4.0.4 alone and with Visualizer Pro 2.0.1 active.

Tests

New tests/e2e/specs/lazy-render.spec.js:

• race regression: renderer scripts delayed 2s via page.route — fails on the old loader, passes with the fix
• sanity: same flow without delay

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