[v0.4] The compiler reads no environment variable beyond the four of §23.1 (follow-up to #167) #180

Follow-up to #167 (reviews of #157 and #163). SPEC.md:8077 says that four operational variables exist and change no output. The production compiler also reads:

• LEXLEAN_AXDBG (crates/lexlean/src/production/certificate/data_a.rs:366), which prints axioms to stderr;
• LEXLEAN_SHOW (:383), which prints a row;
• LEXLEAN_SHOW_FILE (:385-386), which writes that row to a path the environment names.

The SP-17 allow-list sanctions the printout (crates/conformance/src/lean_literals.rs:73).

Definition of done:

• The three hooks and the allow-list entry are removed.
• A gate fails on any std::env::var in crates/lexlean/src that names a variable outside §23.1's four, with a planted defect recorded in VERIFICATION.md.
• just vv green.

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