docs: stop hiding snippet fallback prematurely (:defined race)

[adamziel]

Summary

Reports of "no snippets visible" on https://wordpress.github.io/php-toolkit/reference/html.html despite the page shipping 9 `` elements + 9 `<pre class="snippet-fallback">` static fallbacks.

Root cause: the CSS rule

php-snippet:defined .snippet-fallback { display: none; }

matches the moment `customElements.define('php-snippet', ...)` runs in Playground's module — before the snippet's shadow DOM has any content. In the gap between "custom element registered" and "shadow DOM populated with code + Run button," the fallback was already hidden and the widget hadn't drawn yet. On a slow client (or one where a later step in the Playground module fails) the user sees a permanent blank gap.

Fix: drop the `:defined` rule from `style.css` and hide the fallback explicitly from `page.js`'s `patchSnippet`, which only runs after `shadowRoot.querySelector('pre code')` returns a real node. By that point the snippet's UI is rendered on screen, so swapping the fallback out is safe. If the cross-origin Playground module never loads at all (CSP, adblock, slow network, no-JS), `patchSnippet` is never reached and the fallback stays visible — readers always see the code.

Verification (headless chromium screenshots)

Scenario	Before	After
Module reachable	Custom element renders, fallback briefly invisible during shadow-DOM render	Custom element renders + Run button; fallback hidden only after shadow DOM is populated
Module unreachable (`--host-rules="MAP playground.wordpress.net 127.0.0.1:1"`)	Permanent blank gap because `:defined` matched the moment the module's first script ran	Static ` ` fallback stays visible, readers can read the code

87/87 snippets still match captured stdout. Asset version bumped (`20260503-script-unescape` → `20260503-fallback-explicit-hide`) so browser caches refresh.

Test plan

• `Verify docs snippets` workflow passes.
• After deploy, every reference page on https://wordpress.github.io/php-toolkit/reference/ shows code in every snippet — either as the interactive widget (when the module loads) or as the static `

  ` fallback (when it doesn't). No blank gaps.

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