STDLIB-04b: Throws extern error<T> → real implementation #329

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

Scope

extern fn error<T>(msg: String) -> T / Throws;

declared in stdlib/effects.affine:29.

Current state (audit)

Per the 2026-05-24 audit, error<T> is missing in all backends — no wiring found in lib/interp.ml, lib/js_codegen.ml, or lib/codegen_deno.ml. Sibling panic is fully wired; error should mirror it but return the polymorphic T (i.e. panic-equivalent semantics, divergent — T is uninhabited at runtime because the call doesn't return).

Acceptance

• error<T>(msg) lowered on all backends (matches panic runtime behaviour: throw/abort)
• Throws effect tracked at the call site (parity with panic)
• Hermetic e2e test: error<Int>("…") in a try/handler-equivalent boundary
• CAPABILITY-MATRIX + TECH-DEBT updated in the same PR

Severity / class

S3. Refs #175.