(WIP) MegaMerge from AC main 0.14.1

[softhack007]

This is the work-in-progress megamerge PR, to sync mdev with the latest source code from upstream.

Due to high volume of changes, this PR has a more complicated structure and needs several steps.

• megamerge_for_mdev is a temporary merge target. Ultimately it will be merged into mdev. Here we can cherry-pick commits from upstream to reduce conflicts.
• megamerge_from_ACmain is the merge origin branch. Here we add commits to resolve conflicts.

🚩 don't worry, the build errors are normal. I'm trying to incrementally resolve conflicts file-by-file.

[softhack007]

I'll do a fresh start later.