An already-loaded configuration snapshot to use verbatim.
Present → nothing is read from the environment or from disk, the three
options above are ignored, and this exact object becomes the application's
CAPABILITIES.CONFIG service. Absent → configuration is loaded normally.
This exists so configuration can be resolved BEFORE plugins are constructed — deciding which plugins to register from a value in the environment — without the application then loading a second snapshot. Two snapshots read at different moments can disagree, and the one the composer branched on would not be the one handlers read.