Harden placeholder list persistence, and persist before writing placeholder

(Copying from 1196804 in old backlog)

Currently VFS4G can lose track of a placeholder if either:

1. There is a power loss before VFS4G attempts to write to the placeholder list (but after it has written the placeholder)
2. There is a power loss before the placeholder list is flushed to disk

To address these issues:

1. VFS4G should write to the placeholder list before writing the placeholder
2. VFS4G should flush the file buffers for the placeholder list file

Start by locating VFS4G's placeholder-list persistence and placeholder-write paths; the issue does not name specific files or tests. Trace the power-loss ordering and buffer-flush behavior, then verify that the placeholder list is persisted before the placeholder and flushed to disk.