Problem:
`zipfile://{archive}::{entry}` is ambiguous: `::` is legal in both an
archive path and an entry path, so the separator cannot be identified.
Splitting at the last `::` reads archives correctly but breaks entries
that contain it, and the plugin then emits buffer names it cannot read
back.

Solution:
Address entries as `zip://{archive}/{entry}`, joining the two paths.
Resolve the split by walking components: the first one that is a regular
file is the archive, because a regular file cannot have children on disk.
Entry paths may then contain any character, and no escaping is needed.

zipPlugin.vim keeps its own scheme, so the Java ftplugin emits whichever
form matches the active plugin until the legacy package is removed.

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`.