fix(interp): wire missing string_length builtin (Refs #332, #329)

`string_length : String -> Int / Pure` is declared as a primitive in three
places that AffineScript programs traverse during the compile pipeline:

  * stdlib/effects.affine    — the surface declaration
  * lib/resolve.ml           — recognises the name during resolution
  * lib/typecheck.ml         — types it as String -> Int / EPure

…but the runtime `VBuiltin` entry in lib/interp.ml's
`create_initial_env` was missing. Programs calling `string_length`
compiled cleanly and failed at run with:

  Unbound variable: string_length

This broke two E2E test suites on main:

  - E2E STDLIB-04e Pure #332 / "string_length(\"hello\") == 5"
  - E2E STDLIB-04b error #329 / divergence assertion
    (`lookup` used `string_length(k) > 0` as a guard — the missing
     binding produced the wrong eval_error variant, so the test
     `RuntimeError "empty key"` assertion saw an Unbound variable
     instead.)

Wire the builtin as `String.length` of an OCaml `VString`. Matches the
codegen-Deno path (`(s).length`) and the resolve/typecheck layers'
existing assumptions about the surface.

Found via affinescript#361's build-job CI red after the bench/dune +
fixture-exemption fixes cleared the other two baselines; this stdlib
bug was latent under those failures.

Refs #332 (STDLIB-04e pure externs), Refs #329 (STDLIB-04b throws extern).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

lib/interp.ml

@@ -623,6 +623,20 @@ let create_initial_env () : env =
       | [VFloat f] -> Ok (VString (string_of_float f))
       | _ -> Error (TypeMismatch "float_to_string expects Float")
     ));
+    (* STDLIB-04e (Refs #332): `string_length : String -> Int / Pure` is
+       declared in stdlib/effects.affine, resolved by lib/resolve.ml, and
+       typed by lib/typecheck.ml — but the runtime VBuiltin entry was
+       missing here. Programs that call `string_length` would compile
+       cleanly and fail at run with `Unbound variable: string_length`
+       (broke E2E STDLIB-04e #332 + STDLIB-04b #329 — the latter uses
+       `string_length(k) > 0` as a guard, so the missing binding masked
+       the divergence-vs-RuntimeError assertion as an Unbound variable
+       error instead). Wire it as the canonical String.length surface. *)
+    ("string_length", VBuiltin ("string_length", fun args ->
+      match args with
+      | [VString s] -> Ok (VInt (String.length s))
+      | _ -> Error (TypeMismatch "string_length expects String")
+    ));
     (* STDLIB-04e (Refs #332): `string_to_int` is the typed-alias surface
        declared in stdlib/effects.affine. Same semantics as `parse_int`
        (the canonical name) — the alias was unwired before this PR, so