Function DeepMergeConfigs

Deep-merges configuration layers, EARLIEST first — later layers win per key.

Merge rules (per key):

• plain object vs plain object → recursive deep merge;
• anything else (arrays, strings, numbers, booleans, null) → the later value REPLACES;
• null is a real JSON value and replaces like any primitive;
• undefined values in a later layer are skipped (they cannot exist in parsed JSON anyway).

null/undefined LAYERS are skipped entirely. The inputs are never mutated; the result is a fresh object graph.