/// Identity of the bytes being lexed. Threaded through every emitted `Span`
/// (via `path`). Cheap to clone: `path` is `Arc<str>`.
/// Where the source bytes were obtained - determines what callers can do
/// beyond display:
///
/// - `File`: parent dir known, relative `file=...` resolves, safe to cache
/// by `(path, mtime)`.
/// - `Stdin`: one-shot, no parent dir, no cache.
/// - `Inline`: tests / REPL / doc-test snippet; `path` is synthetic.
/// - `Memory`: LSP unsaved buffer; `version` is the source of truth.