Atomic overlay file application in wwclient

Update wwclient such that each file in the runtime overlay is applied
atomically.

- Closes: #1307
- Closes: #1975
- Closes: #1226

Also adds --once flag to allow wwclient to be run one time.

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

CHANGELOG.md

@@ -4,6 +4,21 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
+## Unreleased
+
+### Added
+
+- `wwclient --once` prompts wwclient to run once. #1226
+
+### Changed
+
+- `wwclient` places files from the runtime overlay atomically. #1307, #1975
+
+### Fixed
+
+- Fixed a bug in `wwclient` which prevented proper cleanup of ephemeral files.
+- Fixed `wwclient --debug` to properly enable debug logging.
+
 ## v4.6.3, 2025-08-01
 
 ### Added