STDLIB-04e: Pure externs (int_to_string / string_to_int / string_length) — close-as-done

[hyperpolymath]

Tracking issue for STDLIB-04e — row-split of STDLIB-04 (docs/TECH-DEBT.adoc:162, Refs #175).

Scope

extern fn int_to_string(n: Int) -> String;
extern fn string_to_int(s: String) -> Option<Int>;
extern fn string_length(s: String) -> Int;

declared in stdlib/effects.affine:32-34.

Current state (audit)

Per the 2026-05-24 audit, all three are real + tested: wired in lib/interp.ml (615-633) and lib/codegen_deno.ml (263-272), with golden + e2e coverage already in the gate.

Acceptance

• Confirm test coverage (point to the existing golden/e2e tests)
• Mark this row DONE in TECH-DEBT.adoc + CAPABILITY-MATRIX in the closing PR
• No code change expected; this is paperwork

Severity / class

S3 — bookkeeping. Refs #175.