chore(stdlib): STDLIB-04c — remove dead string_concat extern (Closes #330)

The `extern fn string_concat(s1: String, s2: String) -> String;` declared
in `stdlib/effects.affine:35` was dead surface: never wired in any
backend (no entry in `lib/interp.ml` builtins or `lib/codegen_deno.ml`
deno_builtins) and never called from any stdlib, test, or fixture file.

The canonical surface for string concatenation is the `++` operator,
lowered in `Value.binop_string` (interp) and `__as_concat` (Deno
codegen) — one source of truth. Removing the dead extern eliminates
the trap where a caller could `use effects::{string_concat}` and get a
program that compiles but throws "string_concat is not defined" at run.

Updates `docs/TECH-DEBT.adoc` row 04c to DONE per the audit-split
contract.

Closes #330. Refs #175.

Author: claude

docs/TECH-DEBT.adoc

@@ -183,9 +183,12 @@ round-trips + Deno codegen __cell-shape assertion) |S3 |DONE
 (`lib/interp.ml`, `lib/js_codegen.ml`, `lib/codegen_deno.ml`); sibling
 `panic` is wired and `error` should mirror it (divergent `T`) |S3 |open
 — issue #329
-|STDLIB-04c |`string_concat` extern — no direct wiring found; `++`
-operator independently lowered. Decide: remove (operator-only) or wire
-to mirror `++` |S3 |open — issue #330
+|STDLIB-04c |`string_concat` extern — *REMOVED* (Refs #330): the
+canonical surface is the `++` operator (lowered in `Value.binop_string`
+and `__as_concat`); the extern was declared but never wired in any
+backend nor called from any stdlib/test/fixture — dead surface. The
+contract is now operator-only and one-source-of-truth. |S3 |DONE
+2026-05-24 (Refs #330)
 |STDLIB-04d |IO externs (`print`/`println`/`read_line`/`read_file`/
 `write_file`) — wired on all backends but no dedicated hermetic e2e
 tests; test-debt, not impl-debt |S3 |open — issue #331

stdlib/effects.affine

@@ -32,4 +32,8 @@ extern fn error<T>(msg: String) -> T / Throws;
 extern fn int_to_string(n: Int) -> String;
 extern fn string_to_int(s: String) -> Option<Int>;
 extern fn string_length(s: String) -> Int;
-extern fn string_concat(s1: String, s2: String) -> String;
+// `string_concat` removed (STDLIB-04c, Refs #330): the canonical surface
+// for string concatenation is the `++` operator, lowered directly in
+// `Value.binop_string` (interp) and `__as_concat` (Deno codegen). The
+// extern was declared but never wired in any backend nor called from
+// any stdlib/test/fixture file — dead surface.