fix: data integrity risk from unhandled fclose(3) failures

## Problem
Bitcoin-abc has the same security issue described in https://github.com/bitcoin/bitcoin/pull/29307.
`fclose(3)` failure may prevent previously written data from being properly flushed to disk. Current code ignores `fclose(3)` failures, leaving file close errors undetected and unhandled.

## Upstream Fix
PR https://github.com/bitcoin/bitcoin/pull/29307: "util: explicitly close all AutoFiles that have been written"

Start by reviewing the linked upstream Bitcoin PR and searching this repository for AutoFiles that have been written. Trace how those files are currently closed and how fclose(3) failures are handled. Done means written AutoFiles are explicitly closed and close failures are no longer ignored.

Written by the indexing model from the issue text.