fix(codegen): wire string_length lowering — sibling to #362's interp binding

The interp binding for string_length landed in #362 (already on main),
but the wasm-backend codegen still does not know it — `codegen.ml`'s
`ExprApp` dispatch fell through to the "Function or variable not found"
error path. The codegen-test fixtures env_at.affine / arg_at.affine /
env_count_and_at.affine all call `string_length(env_at(0))` etc., so
the dune-runtest codegen-fixture loop fails at code-generation time
even after the resolver / typechecker (#332) and interp (#362) layers
were wired.

AS string layout is `[len: i32][bytes...]` at the pointer the arg
evaluates to — reading the length is one `i32.load` at offset 0. Same
shape as the existing tuple-index / record-field load patterns. Three
lines of effective code in the handler.

Closes the gap for ADR-015 S5 (env_at/arg_at) too: their codegen
emits a length-prefixed AS string, and the fixture tests then need
string_length to read that length back. Both pieces (env_at/arg_at
wiring + string_length codegen lowering) are required for the
fixtures to compile end-to-end.

Refs #332 (STDLIB-04e originating ticket), Refs #361 (queue-clearance
incident), follow-on to #362 (interp side) and #364's env_at/arg_at
wiring earlier in this PR.

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

Author: hyperpolymath

lib/codegen.ml

@@ -908,6 +908,17 @@ let rec gen_expr (ctx : context) (expr : expr) : (context * instr list) result =
         in
         Ok (ctx_with_heap, code)
 
+      | ExprVar id when id.name = "string_length" && List.length args = 1 ->
+        (* STDLIB-04e (#332) wasm-backend lowering. AS string layout is
+           `[len: i32][bytes...]` at the pointer the arg evaluates to —
+           reading the length is one i32.load at offset 0. The interp
+           binding (lib/interp.ml) was wired in #362; this handler is
+           the codegen sibling so tests/codegen/*.affine fixtures that
+           call string_length (env_at / arg_at / env_count_and_at) can
+           compile end-to-end. *)
+        let* (ctx_with_arg, arg_code) = gen_expr ctx (List.hd args) in
+        Ok (ctx_with_arg, arg_code @ [I32Load (2, 0)])
+
       | ExprVar id when (id.name = "env_at" || id.name = "arg_at")
                         && List.length args = 1 ->
         (* ADR-015 S5 (#180): env_at(i) / arg_at(i) — fetch the i-th