docs(readme): document source-code locations and build tools for the bundled editor

[erseco]

Summary

WordPress.org's detailed plugin guideline 4 requires public, maintained access to the source code and build tools behind any minified/compiled code — satisfied either by shipping the sources or by "a link in the readme to the development location".

This plugin bundles the compiled eXeLearning editor (dist/static: minified JavaScript bundles, compiled stylesheets and compressed data files), but readme.txt only linked this repository — not the editor's source. This is one of the compliance items for the WordPress.org submission tracked in exelearning/exelearning#1542, and complements the size-reduction work in exelearning/exelearning#2260 (which relies on this link for its dist-side minification of first-party editor code).

Changes

readme.txt gains a == Source Code == section that:

• states that the plugin's own PHP and JavaScript ship as human-readable source in the package (they do — the block and admin scripts are plain source, no build step);
• links https://github.com/exelearning/exelearning as the development location of the bundled editor, names the build entry point (make build-static, Bun, scripts/build-static-bundle.ts), and describes the compressed data files (*.json.zst / *.json.gz curricular datasets and editor data, decoded in the browser by the bundled fzstd library);
• points at the release provenance already shipped in every package: .editor-version (bundled editor version) and dist/static/.build-commit (exact source commit) — both verified present in the v4.0.3 release package and not excluded by .distignore;
• documents how release ZIPs are assembled (make package → make build-editor + wp dist-archive).

A changelog bullet is added under the unreleased 0.0.0 block (version placeholders are rewritten by make package, which this change does not touch).

Notes for review

• Facts double-checked against the packaging pipeline: Makefile writes dist/static/.build-commit during build-editor and stamps Stable tag at package time; .distignore ships dist/static verbatim.
• No code changes; nothing for phpcs/phpunit. The readme parser treats == Source Code == as a custom section, which is the form the Plugin Review Team recognizes for guideline-4 compliance.

[github-actions]

Test in WordPress Playground

Test the plugin with the code from this branch:

ℹ️ The eXeLearning editor is fetched from the shared release and unpacked into the plugin when the playground boots, so the first load may take a few extra seconds. ELP upload, shortcode, Gutenberg block and preview work normally.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.87%. Comparing base (e387742) to head (7298535).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #101   +/-   ##
=========================================
  Coverage     96.87%   96.87%           
  Complexity      864      864           
=========================================
  Files            39       39           
  Lines          4323     4323           
=========================================
  Hits           4188     4188           
  Misses          135      135           

Flag	Coverage Δ
javascript	95.70% <ø> (ø)
php	97.25% <ø> (ø)

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