Problem/Motivation
Split off from #2543334: Auto-placeholdering for #lazy_builder with bubbling of contexts and tags to make that one more easily reviewable. Details to follow.
Proposed resolution
Details to follow.
Remaining tasks
Details to follow.
User interface changes
None.
API changes
None.
Data model changes
None.
Why this should be an RC target
From #10:
Without this, max-age=0 blocks will not be placeholdered automatically, which means that any page with uncacheable blocks will not be able to use Dynamic Page Cache.
This would therefore IMO be a great RC target.