Restore environment_value after source layout change

[isomorphisms]

Boundary

Restores the environment_value : String → IO (Maybe String) surface from #64 in the current buildable _ source layout. The later layout rewrite dropped the merged implementation and its receipts.

Receipts

• present value returns Just value
• an empty value remains distinct from an unset value
• an unset value returns Nothing
• names containing = or NUL are rejected as Nothing

Chez receipt:

hello
empty
missing
missing
missing

This is the same narrow 50-line implementation/test boundary as #64, relocated onto the current tree.