fix(core): keep MAST merge from growing stripped forests

[huitseeker]

This fixes a case where MastForest::compact() could make a stripped forest larger instead of smaller.

The root cause is twofold. First, merge rebuilt basic blocks from the padded operation iterator, so padding ops became "organic" serialized ops. Second, after debug info was cleared, compact still wrote empty node decorator rows for every merged node.

The fix keeps the original basic-block batches during merge and skips empty node decorator rows. The op decorator CSR is still kept as a valid empty structure after debug info is cleared.

Closes #3035.

[bitwalker]

LGTM! Ping me for a re-review after this is marked ready (if needed)

[greenhat]

LGTM! Thank you for a prompt response!

[bitwalker]

@huitseeker Maybe we should apply this to main so it can go out in 0.22.2? It looks like it should be a non-breaking change unless I missed something

[huitseeker]

@bitwalker Is this a blocker for something? The improvements in serialization size from this fix are in the low single-digit percentages. I agree there is no semver issue, but I am not sure if it's worth the trouble.

[bitwalker]

We were observing double-digit reduction in size due to compaction previously on some of our example programs (e.g. 18k -> 13k in one of them), but obviously that can vary significantly. In any case, without compaction, it's a lot easier to hit the account/note code size limits when compiling Rust programs - so it's not a blocker for some feature, but it does directly impact whether Rust components that run up against the limit will be usable or not. Since users won't get stuff in next for quite some time (at least a month?), it's probably worth it.

Since we also have at least one other fix queued up in main, it probably makes sense to include this if it isn't going to be a major pain to do so. I was thinking the divergence between main and next in this code wouldn't be too bad since we just did a release, but you'd have a better idea than me whether this would be annoying to backport.