Problem:
There is no health check. Encrypted entries cannot be read at all, since
Info-ZIP takes a password only from a terminal and `-P` would expose it
in the process arguments.
Solution:
Add `:checkhealth nvim.zip`, reporting the backend and which
implementation is handling archives. Prompt for the password on a pty,
extracting to a file so the entry's bytes stay off the terminal. Report
Info-ZIP's exit code rather than inferring a cause, so an archive using
AES is not reported as a failed decryption.
Problem:
The bundled `zip` plugin is implemented in Vimscript, making it harder to
maintain and build on with Nvim's Lua runtime infrastructure.
Solution:
Add an opt-out `zip.lua` browser backed by `nvim.dir`, and disable the legacy
Vimscript implementation by moving it to `pack/dist/opt/zip/`. Load it with
`:packadd zip`.
