STDLIB-04c: `string_concat` extern → real impl or remove (operator-only)

[hyperpolymath]

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

Scope

extern fn string_concat(s1: String, s2: String) -> String;

declared in stdlib/effects.affine:35.

Current state (audit)

Per the 2026-05-24 audit, string_concat has no direct wiring found in any backend. Concatenation today is the ++ operator, which is independently lowered. So this extern is either dead surface or genuinely unwired.

Acceptance (decide one)

• If genuinely operator-only: remove the extern from stdlib/effects.affine + verify no internal caller breaks; document ++ as the canonical surface.
• If callers exist: wire on all backends (mirror the ++ lowering) + hermetic round-trip test.
• CAPABILITY-MATRIX + TECH-DEBT updated in the same PR.

Severity / class

S3 — small, mostly verification. Refs #175.