Prevent an out-of-bounds terminator write in path_dirname_copy

`path_dirname_copy(dst, 1, "file")` passes the `n + 1` capacity check because `path_dirname_len` returns zero. The fallback then writes both `'.'` at index 0 and a NUL at index 1. A one-byte destination is therefore insufficient despite being accepted. This affects the no-directory fallback, not ordinary nonempty directory copies.

Source evidence at the head of #520:

- [std/path/copy.wave:97](https://github.com/wavefnd/Wave/blob/2ae0a91c57fd76323124c0e4dff552adbfadbe1b/std/path/copy.wave#L97) — `pub fun path_dirname_copy`

Acceptance:

- [ ] Compute the fallback output length before checking capacity; reject insufficient capacity before any destination write.
- [ ] Add guarded-buffer cases for capacities 0, 1 and 2, empty input and a simple filename. Capacity 2 must produce a NUL-terminated dot.
- [ ] Keep root and ordinary directory results unchanged. Tests must check guard bytes, not only return values.

Audit status: identified by static source inspection; the scenarios above have not been executed during this audit. The permalink fixes the reviewed revision; this report does not claim the defect was introduced by #520.

Start at std/path/copy.wave:97, the path_dirname_copy entry point, and inspect the capacity check and no-directory fallback. Add or run guarded-buffer cases for capacities 0, 1, and 2 with empty input and a simple filename, then verify guard bytes and that root and ordinary directory results remain unchanged.

Written by the indexing model from the issue text.