Use released compile-extension package for WASM builds

[adamziel]

What changed

• Removes the WordPress/wordpress-playground sparse checkouts and shared base-image artifact job from the WASM workflows.
• Builds through the released @php-wasm/compile-extension@3.1.28 package, including --prepare-image for the Rust builder base image.
• Replaces the old Playground-internal loader smoke test with released @php-wasm/node@3.1.28 and @php-wasm/universal@3.1.28 packages.
• Updates the wasm-spike docs/comments to describe the package-only flow.

Why

@php-wasm/compile-extension now lazily fetches the Docker assets it needs from its matching Playground release, so downstream CI no longer needs to shallow or sparse checkout wordpress-playground just to build this extension.

Validation

• bash -n packages/php-ext-wp-mysql-parser/wasm-spike/build-in-docker-rust.sh
• node --experimental-wasm-jspi --check packages/php-ext-wp-mysql-parser/wasm-spike/run-spike.mjs
• git diff --check
• Parsed both updated workflow YAML files with yaml@2
• Confirmed @php-wasm/compile-extension, @php-wasm/node, and @php-wasm/universal all publish 3.1.28
• Confirmed runtime package imports install cleanly

I could not run the full Docker build locally because Docker is not installed in this environment; the PR workflow should cover that path.