[v25.3.x] iceberg: bring back in-manifest stats

Backport https://github.com/redpanda-data/redpanda/pull/31332 to branch v25.3.x. This issue was created as a placeholder because the original PR's commit(s) could not be automatically cherry-picked. Here are the commands to execute: ``` git checkout v25.3.x git checkout -b manual-backport-31332-v25.3.x-190 git cherry-pick -x ddb4ace072 fcad9e1362 f3cfe07a0e 9a98ac5ee1 e12338af5b cb7ebf5905 5497e8fc15 3eaf8279bc babc950f87 5e64e28bf6 git push origin manual-backport-31332-v25.3.x-190 gh pr create \ --title "[v25.3.x] iceberg: bring back in-manifest stats" \ --base "v25.3.x" \ --label "kind/backport" \ --head "manual-backport-31332-v25.3.x-190" \ --draft \ --repo "redpanda-data/redpanda" \ --milestone "v25.3.x-next" \ --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/31332 "