Add Cmd.check_available!

[dmisiuk]

Adds Cmd.check_available!, closes #223 — check whether a command can be found on the system before trying to run it.

Discussed on Zulip. @lukewilliamboswell approved a pure-Roc implementation on the existing primitives, so there are no host or glue changes.

Implementation

check_available! : Str => Bool

• Reads PATH via Env.var! and splits it byte-preservingly through OsStr.to_raw, so non-UTF-8 entries survive.
• Branches on Env.platform! for the :/; separator and, on Windows, the candidate extensions from %PATHEXT% (a name is tried as-is and with each extension appended).
• A name containing a path separator is checked directly instead of being looked up.

Directories are excluded. Path.is_executable! (Unix) and Path.exists! (Windows) both return true for a directory, so a candidate is additionally required to not be a directory. Because is_dir! does not follow symlinks, a symlink to an executable still resolves while a real directory named like the command is rejected. The only residual is a symlink pointing at a directory, which is documented. (The triage note asked to avoid the directory failure mode; this does, in pure Roc.)

Testing

Six expect blocks cover the pure core — PATH splitting for all three OsStr representations, byte preservation of a non-UTF-8 entry (verified to fail when the bytes are altered), empty-entry dropping, and separator detection.

Verified end-to-end against a local build with a directory, a symlink, and a real executable sharing a name:

realtool (executable file) -> available
faketool (directory)       -> missing
linktool (symlink to exe)  -> available

examples/check-command.roc demonstrates the API and its test_spec.json case asserts a real command is found and a bogus one is not. Full ./scripts/test.py passes — host build plus all examples. The example's run case is Unix-only (like command.roc), since it checks a Unix command by name.