Ignore trailing separators when deriving path basenames

`path_basename_start` updates the basename start after every path separator and does not skip separators at the end of a path.

As a result, paths ending in one or more separators produce an empty basename even when a final path component exists before them.

For example, the current source-derived behavior is equivalent to:

- `dir/` → empty basename
- `/usr/bin/` → empty basename
- `/usr/bin///` → empty basename

`path_dirname_copy` already provides component-oriented path behavior, so basename handling should not lose the final non-separator component merely because the path contains trailing separators.

Code evidence:

- `std/path/analyze.wave` — `path_basename_start`, `path_basename_len`
- `std/path/copy.wave` — `path_basename_copy`

Acceptance:

- [ ] Ignore trailing path separators when locating a non-root basename.
- [ ] Add cases for `dir/`, `/usr/bin/`, and repeated trailing separators.
- [ ] Define and preserve explicit behavior for root-only paths such as `/`.
- [ ] Keep both `/` and `\\` separator behavior covered.
- [ ] Do not regress paths without trailing separators.

Start by reading path_basename_start and path_basename_len in std/path/analyze.wave, then compare their behavior with path_basename_copy in std/path/copy.wave. Add regression cases for dir/, /usr/bin/, repeated trailing separators, both slash types, root-only paths, and paths without trailing separators. Done means trailing separators are ignored for non-root basenames while root behavior remains explicit and unchanged.

Written by the indexing model from the issue text.