Normalize an absolute filesystem path

Question

Read one absolute path and return its canonical form: empty segments and `.` segments disappear, `..` folds into the parent, and exactly one slash separates the surviving directories.

```python
def solve(raw: str) -> str:
    ...


def run_case(case: dict) -> str:
    return solve(case["stdin"])
```

## Requirements

- The input is a single absolute path; it may repeat separators and may end with a slash.
- A whole segment equal to `.` is dropped; only a whole segment equal to `..` means "go up".
- `..` removes the most recently kept segment; when nothing is left to remove it has no effect, so climbing past the root stays at the root.
- Join the remaining segments with one leading slash, no doubled slashes, and no trailing slash.
- Return `/` alone when no segment survives.
- Return a `str` with no surrounding whitespace; symlinks and permission checks are out of scope.
- Do not append a trailing newline to the returned string; return the value exactly once.