Dangling symlink at the destination escapes conflict detection

[darkone-linux]

Summary

A dangling symlink sitting at the destination path is invisible to the
conflict detection, because Path::exists() follows the link and answers
false when the target is missing. Two consequences, one cosmetic and one that
can destroy the symlink.

1. Confusing error message (Linux)

$ ln -s nowhere.txt cafe.txt     # dangling
$ printf 'x' > 'Café.txt'
$ rename-simple 'Café.txt'
[E] .: Café.txt -> File exists (os error 17)

Nothing is lost — renameat2(RENAME_NOREPLACE) refuses the rename, exactly as
designed — but the raw errno leaks out instead of the usual
File name already exists. The op slipped past filter_conflicts
(src/main.rs:328, exists() → false) and only got caught at the syscall,
where the error is reported verbatim (src/main.rs:414).

2. The dangling symlink is silently replaced (non-Linux)

On macOS/BSD/Windows, rename_no_clobber falls back to a try_exists()
pre-check followed by std::fs::rename (src/main.rs:392-401).
try_exists() follows the link too, so it also answers false for a dangling
symlink, and fs::rename on Unix then overwrites it. The symlink is
destroyed without a word — the destination looked free but was not.

This contradicts the guarantee stated in the README and the man page: an
existing destination is never overwritten.

Suggested fix

Both come from the same root cause: existence is tested through the link
instead of on the entry itself. symlink_metadata() answers about the entry —
it is what compare_entries already switched to in 0.7.1 (src/lib.rs:739).

• In filter_conflicts, detect the destination with
  to.symlink_metadata().is_ok() rather than to.exists(), so a dangling
  symlink is reported as a normal conflict with the usual message and never
  reaches the rename.
• In rename_no_clobber, use the same check in the non-Linux fallback, closing
  the overwrite. The Linux path already refuses at the syscall level.

compare_entries needs no change: a symlink on either side already answers
NotComparable, so -D will not touch it.

Worth adding a tests/unix_tests.rs case asserting that a dangling symlink at
the destination survives, and that the error message is the regular one.

Found during the code review that led to 0.7.1. Pre-existing on all platforms.