fix(op-node/derive): channel size incorrect when pruning frames after last frame

## Problem

In `Channel.AddFrame`, when the closing frame arrives and frames with index `>= endFrameNumber` are pruned, `ch.size` was decremented for every entry iterated in the map, not only for frames that are actually deleted.

## Expected

Decrement `ch.size` only when a frame is removed (same branch as `delete(ch.inputs, idx)`).

## Related PR

Will be fixed by the linked PR (see PR description).

Start at Channel.AddFrame in op-node/derive and inspect the closing-frame pruning path for indexes >= endFrameNumber. Verify that ch.size changes only when an entry is deleted from ch.inputs, then run the relevant derive tests and confirm the channel size remains correct when non-deleted frames are iterated.

Written by the indexing model from the issue text.