Problem:
Several subsystems need to derive a short, filesystem-safe identifier from an
arbitrary path, and each reinvents it ad-hoc:
- `'undodir'` and `swapfiles` encode the full path into a single filename, which
may exceed filesystem length-limits.
- `:connect ssh://` needs the SSH ControlPath socket name to stay under the
104-byte `sun_path` limit on macOS; today the path overflows it.
- the upcoming :terminal state dir.
- arbitrary plugin purposes.
Solution:
Provide `vim.fs.slug()`, which generates a bounded, one-way filename from an
arbitrary string. The input is normalized so equivalent paths produce the
same result. An 8-char hash is appended for uniqueness
7b03df5d54