Add Idriç environment value lookup

[isomorphisms]

Adds environment_value : String -> IO (Maybe String) beside the existing process operations in System.

• rejects names containing = or embedded NUL
• preserves the distinction between unset and empty values
• copies the result through Idriç String construction before returning
• adds focused .idric tests for present, empty, absent, and invalid names

Execution receipt (Chez backend):

hello
empty
missing
missing
missing

Status: PASS. Both acceptance invocations were compiled from .idric source and run; IDRIC_ENV_TEST=hello printed hello, and env -u IDRIC_ENV_TEST printed missing.

[isomorphisms]

Classification: READY_TO_MERGE.

Retargeted from the merged historical idric-source-extension branch to the current Idriç branch. The old base is an ancestor of Idriç; the PR remains a one-commit, five-file environment-value change with the same 50-line diff, and a local three-way merge against current Idriç is conflict-free.