[007xf]

Fix two reproducible errors:

• Cache validation omitted the application data parent itself. Replacing that parent with a symlink after initialization could delete files in an external Cache directory. Include that parent in validation; regression checks preserve an external sentinel and reject external cache statistics.
• WebDAV health searched for text such as "200 OK" anywhere in the body, so a filename could make a failed property response appear ready. Parse complete DAV XML and accept only a successful property status at the expected element path. Cover misleading filenames, truncated XML, unrelated namespaces and a valid default DAV namespace.

Validation: both failure regressions failed against the original implementation and pass after the fix. Core smoke checks, account-free application lifecycle and single-instance/crash-recovery checks pass. An Apple Silicon release build and arm64 architecture verification pass using the compatible SDK already used by repository scripts. The local macOS 27 SDK lacks its SwiftUI macro plugin. No real Quark credentials or network-drive account were accessed; real Infuse playback and third-party engine range reads remain unverified.