Updates to handle variable Stan_Cases

Add caseRoot: URL? parameter to all public commands (v1.1.0)

Adds a backward-compatible `caseRoot: URL? = nil` parameter to every
public command function and to `casePaths(for:)`. When non-nil the
supplied URL is used as the case root directly, bypassing the
STAN_CASES env var and ~/Documents resolution entirely. Existing
call sites with no caseRoot argument are unaffected (default nil
preserves the previous behaviour).

Also adds a `casePaths(for:root:)` overload as the new implementation
seam; the existing zero-arg `casePaths(for:)` forwards to it with
root: nil.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>