install.sh takes a mistyped version as the install directory #1409

Problem

install.sh decides whether an argument is a version by matching X.Y.Z, latest or beta; anything else is a directory. A version typed with a v prefix, as release tags are often written, becomes a folder.

./install.sh v0.52.0

Output: > Downloading the latest version: '0.52.0', > bashunit has been installed in the 'v0.52.0' folder, exit 0. A directory named v0.52.0 now holds the latest release, and no error was printed.

Where

• install.sh:164-166: is_version accepts only ^[0-9]+\.[0-9]+\.[0-9]+$, latest and beta.
• install.sh:169-174: with one argument, a non-version is the directory.

Why it matters

A CI step pinning v0.51.0 silently installs the current release into a directory the job never looks at, and the next step fails on a missing binary or, worse, picks up an unpinned version from elsewhere on PATH.

Suggested change

Treat an argument that looks like a version with a prefix (v0.52.0, 0.52) as a version and either accept it (strip the v) or refuse it with Error: unknown version 'v0.52.0'; use 0.52.0, latest or beta. Keep the free-form directory for arguments that do not look like a version at all.

Acceptance criteria

• ./install.sh v0.52.0 installs 0.52.0 into lib, or exits 1 naming the version; it never creates v0.52.0/.
• ./install.sh lib v0.52.0 behaves the same way.
• A test covers it and fails on the current code.

Proposed fix

In install.sh, after is_version (install.sh:164-166), add looks_like_version that matches ^v?[0-9]+(\.[0-9]+){0,2}$ with the existing regex_match helper (the script already uses [[ =~ ]] there). In the argument parsing at :168-186, an argument that looks like a version but is not one accepted by is_version is an error, never a directory:

function looks_like_version() {
  regex_match "$1" '^v?[0-9]+(\.[0-9]+){0,2}$'
}
# in the parse, before treating an argument as DIR:
if looks_like_version "$1"; then
  echo "Error: unknown version '$1'. Use a release such as $LATEST_BASHUNIT_VERSION, 'latest' or 'beta'." >&2
  exit 1
fi

Alternatively strip a leading v and accept v0.52.0 as 0.52.0; the error is the smaller change and avoids guessing. Bash 3.0: [[ =~ ]] is already in the file through regex_match; keep the pattern free of quotes and backslashes so 3.0 and 3.2 read it the same. Public behaviour that must not change: ./install.sh lib, ./install.sh 0.52.0, ./install.sh lib 0.52.0, latest and beta. Docs: docs/installation.md:76 can say the version is written without a v; add a CHANGELOG line under Unreleased.

Agent brief

• Branch: fix/1409-install-rejects-mistyped-version from fresh origin/main. Commit: fix(install): refuse a version that is not a release instead of using it as a directory with Closes #1409 in the body.
• Files: install.sh, docs/installation.md, CHANGELOG.md. Test: extend tests/acceptance/install_test.sh, next to test_install_fails_loudly_on_unknown_version (install_test.sh:128). This case needs no network: the script must exit before downloading.
• RED first:

function test_install_refuses_a_version_with_a_v_prefix() {
  local output
  local exit_code=0
  output="$(./install.sh v0.52.0 2>&1)" || exit_code=$?

  assert_equals "1" "$exit_code"
  assert_contains "unknown version 'v0.52.0'" "$output"
  assert_directory_not_exists "./v0.52.0"
}

Run only that file: ./bashunit tests/acceptance/install_test.sh.

• Non-goals: do not touch the download or checksum code (install.sh removes the installed binary before downloading, so a failed download leaves nothing #1406), the cd (install.sh resolves a relative directory against its own location instead of the current directory #1408) or the beta build.
• Gate, once, when the change is complete: ./bashunit tests/, ./bashunit --parallel --simple --strict tests/, make sa, make lint. make lint is the formatting authority; never shfmt -w; 2-space indent.
• Done when: every acceptance box is checked, the gate is green, CHANGELOG and docs are updated, and a PR is open with the pr skill.