Fix C++ runtime loading issue by correcting js-lzma dependency and paths

[Rhinoatron]

This PR resolves an issue where the C++ environment remains stuck on "Loading..." due to missing js-lzma assets and incorrect file paths.

Root cause:
js-lzma dependency not present after cloning
outdated references to external/js-lzma paths

Changes:
updated all references from external/js-lzma to js-lzma
ensured required dependency is available locally
verified C++ environment initialises successfully

This fix restores full functionality of the C++ runtime environment.

[Rhinoatron]

The C++ runtime issue has been resolved locally and the build now runs successfully. The remaining deploy failure appears to come from the base repository’s pull_request_target workflow, which is still attempting to download a preview artifact that is not available. Because pull_request_target uses the workflow definition from the base branch, workflow fixes made in this PR do not fully apply until this request has been merged.

[kottochii]

As most of these involve changes in the path to js-lzma and creation of the submodule, commits in most files make sense.
There has been one file that looks like product of compilation, which I assume has been placed in the PR by mistake (the only one with a comment on it) - pls delete this, if it is not required.

In terms of changes in the /workflows/, parts of them actually do make helpful changes, but they are not affecting the topic of the PR. Should it perhaps be in another pull request?

[kottochii]

This file looks like a product of compilation. Seems like it has been accidentally added.