The "static" source kind always rendered a SINGLE color and the name
confused new code paths. Rename the module + kind to "single". Storage
keeps backward-compatible serialisation. Frontend color-strip cards /
gradient / index / test modules and the affected tests follow the new
name.
Allow composite sources to reference other composite/mapped sources as
layers. Adds cycle detection (via transitive dependency graph walk),
depth limiting (MAX_COMPOSITE_DEPTH=4), and a runtime safety net in the
stream manager. Frontend layer dropdown now shows all source types
except the source being edited.
17 new tests covering cycles, depth limits, and valid nesting — all
715 tests passing.