Proxpak v2.0.0: rebrand from pve-export-ova, GPL-3.0 re-license

Key changes since v1.1.9:
- Project renamed: pve-export-ova -> proxpak (folder, script filename,
  runtime paths like /var/tmp/proxpak-lvm.*, LV names proxpak-tmp-*)
- License: MIT -> GNU GPL-3.0-or-later
- Script header includes GPL-3.0 license block
- Runtime warranty notice prints to stderr on every invocation
- Prominent export-location banner replaces low-key OK message
- README: new Post Export Workflow section with mermaid diagram and
  WS Pro 17 caveat; new Comparison with Similar Tools section
- Strict ASCII punctuation pass across docs and comments
- Inline intent comments added above 7 major functions

Note: .pve-export-meta sentinel filename preserved for backwards
compatibility with v1.x export directories already on disk; --cleanup
against those will still work.