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.

Pre-rebase provenance

Before branch-history cleanup, exact head 0625afbd0d4976485ba766c5c3d3e97ec0099fbb passed all four current-head workflows:

• Idriç build — run 35236799970
• Idriç source-layout check — run 35236800103
• Device action target matrix — run 35236799962
• Idriç source-style drift check — run 35236799964

These runs remain evidence for the pre-rebase exact head. They are retained as provenance and are not claimed as verification of any rewritten head.