Add an opt-in FileAccessControl middleware that resolves the warehouse file
and enforces File::canBeAccessedBy() (default public, so existing consumers
are unaffected) when files.access_control.enabled is set. Resolution is
delegated to a configurable files.warehouse.resolver (ResolvesWarehouseFiles)
and the resolved file is stashed on the request so the controller reuses it.
Exposed as the `files.access` route-middleware alias and auto-attached to the
package warehouse route. Adds the FileAccessControl unit suite.
