feat(assert): symlink assertions

[Chemaclass]

🤔 Background

Related #981

Nothing in the catalogue could assert anything about a symlink. Every filesystem assertion follows the link (-f, -e report on the target), so a link and the file it points at were indistinguishable — and a dangling link answered "does not exist", the same as a path that was never created:

ln -s "$dir/target" "$dir/link"
assert_is_file "$dir/link"           # passes — describing the target

ln -s "$dir/missing" "$dir/broken"
assert_file_not_exists "$dir/broken" # passes — describing nothing

That matters for what people actually write bash for: install scripts, dotfile managers, a release layout with a current -> releases/N pointer. For those, "the link exists and points where it should" is the assertion.

💡 Changes

Assertion	Passes when
assert_is_symlink "$path"	$path is a symbolic link, resolved or not
assert_is_not_symlink "$path"	$path is not a symbolic link
assert_symlink_to "$target" "$path"	$path is a link and points at $target

🧭 Two decisions the issue asked to be made, not guessed

A dangling link is still a link. assert_is_symlink passes for one — separating "is a link" from "the target resolves" is exactly the distinction that was missing, and the two existing assertions already answer the second question.

assert_symlink_to compares the target as written, through readlink, not the resolved path. It's what the test author wrote, so a failure names something they recognise; and readlink -f is GNU-only, so resolving would need a second implementation for BSD/macOS. A relative link compares as the relative string it is — documented.

✅ Verification

8 new tests, all RED first. They skip on Windows, where Git Bash turns symlink creation into a copy — matching the existing guard on the permission tests.

docs/assertions.md, both completion scripts and the doc snapshot updated. The completions parity test caught assert_symlink_to missing from the lists, which is the job it exists for.

make sa · make lint · bash build.sh bin -v → ✅ Build verified ✅ · 1705 sequential / 1664 parallel-simple-strict.