fix: harden command paths and filesystem state handling #455

Problem

Candidate/version paths, current-version state, and tests are handled inconsistently, causing unsafe deletion risks, memory leaks, and weak coverage.

Acceptance criteria

• Validate candidate/version inputs as single path components.
• Protect active versions across absolute, relative, normalized, dangling, and copied-current states.
• Remove candidate memory leaks and safely handle empty/non-UTF-8 SDKMAN_DIR.
• Enforce version arguments, provide user-facing errors, and keep help aliases executable.
• Use command-scoped test environments with parameterized cases and filesystem postconditions.
• Preserve valid output and cross-platform behavior without new dependencies or APIs.

Verification

Run formatting, locked checks, strict Clippy, tests, and actionlint; cover traversal, current-link variants, empty configuration, and missing version metadata.