uSync.Complete: v17.4.2 release

Bumped uSync core dependencies to 17.4.1
Cut the v17.4.2 release: paged file push no longer resending the whole file set per page, media rename cleanup on import, multi-pattern file replacement collision fix, PushFiles large-upload handling, plus a batch of sync/export performance work (fewer DB queries, less buffering, cheaper hashing/zip)
Released release/v17.4.2 and published customer-facing notes to the uSync.Complete.Issues repo
Forward-ported v17/main into v18/main (PR #60): the v17.4.2 fixes/perf batch above, the temp-folder cleanup job, and dependency bumps; bumped v18's own uSync pins to 18.1.3 and renamed package-build.yml to prerelease.yml (manual-trigger only) so v18 prereleases can be triggered on demand too
Cut the v18.1.3 release: bumped VersionPrefix, updated CHANGELOG/release notes, tagged release/v18.1.3 and published customer-facing notes to the uSync.Complete.Issues repo