Function write_binding 

pub fn write_binding(
    workspace_root: &Path,
    mem: &str,
    name: &str,
    binding: &Binding,
) -> Result<(), StoreError>

Write a v2 binding to <root>/.memstead/projections/<mem>/<name>.json.

A binding occupies the same per-mem projections tier and file identity its predecessors did (stem-identity preserved), so this overwrites a prior-generation projection file in place when migrating a workspace.