compiler: lower polymorphic len() to wasm (string + array length)

String-wall residual found while verifying the migration corpus is
unblocked: len() did not lower to wasm (Codegen.UnboundVariable "len"),
only string_length did. AffineScript arrays and strings share the same
[len: i32][payload...] header (the list-concat handler already relies on
this), so len() is the same single i32.load at offset 0 regardless of
whether its argument is a String or an array. Merged into the existing
string_length arm.

This unblocks the stdlib string layer on the wasm backend
(starts_with / ends_with / substring / split / join / pad_* all call
len() on strings, and join also on arrays), which the migration corpus's
String.length pattern (30 files) needs.

Verified: len("abcd")=4 and len([10,20,30])=3 lower and run; full
run_codegen_wasm_tests.sh green (no regressions).

Note (separate, not fixed here): string ==/!= still lower to pointer
comparison, not value comparison (string_sub(...) == "ab" is wrongly 0) —
a #555-class interp-vs-wasm divergence, tracked for a follow-up slice. It
only affects string-logic kernels; integer-brain extraction (strings kept
host-side) is unaffected.

https://claude.ai/code/session_01WoKhFQePiRsAj7aqnxbG8s

Author: claude

lib/codegen.ml

@@ -1002,14 +1002,24 @@ 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 ->
+      | ExprVar id when (id.name = "string_length" || id.name = "len")
+                        && 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. *)
+           compile end-to-end.
+
+           `len` (the polymorphic length, issue #135) lowers identically:
+           AS arrays share the same `[len: i32][elems...]` header (the
+           list-concat handler above relies on exactly this), so reading
+           length is the same single i32.load whether `len` is applied to a
+           String or an array. Sharing the arm unblocks the stdlib string
+           layer (starts_with/ends_with/substring/split/join, which call
+           `len` on both strings and arrays) for the wasm backend — the
+           string-wall slice-8 residual flagged in the migration ledger. *)
         let* (ctx_with_arg, arg_code) = gen_expr ctx (List.hd args) in
         Ok (ctx_with_arg, arg_code @ [I32Load (2, 0)])