Add layered config primitives and resilient lazy cache fallback - #33

Summary

This introduces the ArrayKit-side configuration primitives needed by Foundation's lower-library utilization pass without changing existing Config::merge() or ArrayMulti::mergeRecursiveDistinct() semantics.

Changes

• add ConfigMerge for recursive associative-map overlay with atomic list replacement
• add LayeredLazyFileConfig with explicit fallback < source < overrides precedence
• guarantee layered exact-path reads match fully merged namespace semantics, including list/scalar shadowing
• add ResilientLazyFileConfig so corrupt generated namespace caches fall back to authoritative source files
• treat malformed/invalid lazy __flat.php indexes as disposable cache misses
• make Environment::all() include process-only getenv() values with the same $_ENV > $_SERVER > getenv() precedence as direct reads
• add regression coverage for environment enumeration, merge semantics, structural shadowing, and cache corruption fallback
• document the new configuration-layering behavior

Foundation migration

After this lands, Foundation can replace its local ConfigMerger, move fallback/source/override resolution to LayeredLazyFileConfig, and remove its cache-corruption recovery wrapper around ArrayKit lazy config.

Compatibility

Existing merge APIs retain their current behavior. The list-replacing semantics are exposed through the new explicit ConfigMerge API.