Streams the bytes of a referenced file. fileId may be a reference object id or a stored filename.

An authenticated caller receives current working bytes when this exact branch is already active on a Design Engine. This request never starts a project; if no live instance is available it falls back to committed branch HEAD. Anonymous access to a public project always reads committed branch HEAD and never observes working state. Committed snapshots use the immutable assets.primary.sha256 blob, with a legacy files-prefix fallback for older snapshots.