[gwenaskell]

Summary

LogEvent::insert and LogEvent::remove used to invalidate the cached estimated JSON size, so every component that touched a single field paid a full walk of the event at its output.

The cache is now updated from the inserted, replaced and removed values when the target's parent object already exists:

• replace: + new - old
• new member: + "key":value, plus a comma if the object was not empty
• removed member: - "key":value, minus a comma if the object had other members

Inner::clone now keeps the JSON size cache so the update also works after Arc::make_mut.

Fallbacks to invalidation (same behavior as before):

• missing parent or non-container on the path (insert creates intermediates)
• array element as target (padding / negative index semantics)
• remove_prune(.., true)
• root path
• any get_mut / value_mut access

size_cache (ByteSizeOf) is still invalidated.

Not covered: remap, since VrlTarget goes through into_parts / from_parts and drops the cache.

References

Vector configuration

N/A

How did you test this PR?

• Unit tests checking that the cache matches a full recompute after replace / new member / nested (incl. through arrays) / remove / missing-key remove, and that the fallback cases invalidate.
• Quickcheck property test: random nested inserts/removes (small key pool, indices, sometimes on a shared event) and asserts the cached size equals a full recompute after each one. Checked it fails when the comma rule is broken.

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.