Replace babel-loader with swc-loader for webpack transpilation

[rtibbles]

Summary

Replace babel-loader with swc-loader for webpack transpilation and switch TerserPlugin to use SWC's minifier. SWC is a Rust-based compiler that roughly halves the production build time.

Babel is retained in two places:

• Core bundle entry point — SWC supports core-js entry expansion, but has known bugs in its polyfilling behavior that could cause regressions for our supported browsers. Babel's @babel/preset-env with useBuiltIns: 'entry' is more reliable here.
• Jest transforms — SWC's Jest transpiler didn't work out of the box, and Babel is straightforward here. Config and dependencies are moved from kolibri-build to kolibri-jest-config.

References

Builds on the build time improvements in #14457.

Reviewer guidance

• Run pnpm build and verify the production build completes without errors
• Run pnpm test-jest to confirm Jest transforms still work with the moved Babel config
• The SWC minifier options in packages/kolibri-build/src/webpack.config.base.js:117 are worth a close look — the compress/mangle/output options have slightly different semantics than Terser's
• parallel: true was removed from TerserPlugin — it's ignored when using swcMinify since SWC's native code is already fast single-threaded

AI usage

Claude Code was used to implement the SWC migration, with manual review of the generated config changes and verification that builds and tests pass.

[github-actions]

npm Package Versions

Merging this PR will publish the following packages to npm:

Package	Current	New
kolibri-build	1.0.0	1.1.0

Warning

The following packages have changed files but no version bump:

Package	Version	Changed files
kolibri-jest-config	1.0.0	2

If these changes affect published code, consider bumping the version.

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.20.0.dev0_git.20260413195528.pex
Windows Installer (EXE)	kolibri-0.20.0.dev0+git.20260413195528-windows-setup-unsigned.exe
Debian Package	kolibri_0.20.0.dev0+git.20260413195528-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.20.0.dev0+git.20260413195528.dmg
Raspberry Pi Image	kolibri-pi-image-0.20.0.dev0+git.20260413195528.zip
TAR file	kolibri-0.20.0.dev0+git.20260413195528.tar.gz
WHL file	kolibri-0.20.0.dev0+git.20260413195528-py2.py3-none-any.whl

Smoke test screenshot

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[bjester]

I ran pnpm run devserver-hot and make assets, then accessed Kolibri and noticed no issues. It's definitely faster, particularly the production build.

Although, I noticed a couple things possibly worth investigating:

1. It doesn't seem to be necessarily an issue, but I noticed this error running make assets. I don't think it should be loading map files for dependencies? but also it's trying to append a URL to a path or something:

[INFO: Kolibri Build] Completed 'kolibri.plugins.policies.app' bundle in 7.168s!
  ERROR  failed to read input source map: failed to find input source map file "https://cdn.jsdelivr.net/gh/AnonyCo/FastestSmallestTextEncoderDecoder/EncoderDecoderTogether.min.js.map" in "/home/bjester/Projects/learningequality/kolibri/swc-loader-transpilation/node_modules/.pnpm/fastestsmallesttextencoderdecoder@1.0.22/node_modules/fastestsmallesttextencoderdecoder/EncoderDecoderTogether.min.js" file as either "/home/bjester/Projects/learningequality/kolibri/swc-loader-transpilation/node_modules/.pnpm/fastestsmallesttextencoderdecoder@1.0.22/node_modules/fastestsmallesttextencoderdecoder/https://cdn.jsdelivr.net/gh/AnonyCo/FastestSmallestTextEncoderDecoder/EncoderDecoderTogether.min.js.map" or with appended .map
    at crates/swc/src/lib.rs:400

2. Also running make assets, I notice some messages for the mathjax files are not computing the size difference properly and outputting warnings:

[INFO: Kolibri Compressor] Successfully compressed: kolibri/plugins/perseus_viewer/static/mathjax/2.1/jax/output/HTML-CSS/fonts/TeX/WinIE6/Regular/AMS.js by -Infinity %
[WARN: Kolibri Compressor] Compressed size is more than 75% of original size
[INFO: Kolibri Compressor] Successfully compressed: kolibri/plugins/perseus_viewer/static/mathjax/2.1/jax/output/HTML-CSS/fonts/TeX/WinIE6/Regular/Bold.js by -Infinity %
[WARN: Kolibri Compressor] Compressed size is more than 75% of original size

[rtibbles]

It doesn't seem to be necessarily an issue, but I noticed this error running make assets. I don't think it should be loading map files for dependencies? but also it's trying to append a URL to a path or something:

Yes - when the source map for the file is hosted on a CDN it doesn't seem to work properly - this was being silently dropped by babel previously, and I couldn't work out how to suppress the warning here.

Also running make assets, I notice some messages for the mathjax files are not computing the size difference properly and outputting warnings:

Interesting - I think I can look at this further within the scope of #14388 where these files are dropped in favour of a newer version of Mathjax.

I ran pnpm run devserver-hot and make assets, then accessed Kolibri and noticed no issues. It's definitely faster, particularly the production build.

Noting that because we dropped babel transpilation from the dev build - this will make no difference there! But yes, on the production build it should be a noticeable speedup.

[bjester]

Cuts the production build time in half! My concerns have been acknowledged