fix(typecheck): make len/show truly polymorphic schemes (Refs #122)

v2.5/v2.6 (merged in #123) bound `len` and `show` via `bind_var` as
`'a -> Int` / `'a -> String` using a SINGLE SHARED unification tyvar,
not a generalized scheme. The first use pinned it (e.g. `len(name)`
=> tv:=String), so a later `len(ids)` on `[String]` failed with
`Unify TypeMismatch (String, Array[String])`. This blocked any program
mixing `len` over strings and arrays — notably the ubicity#30 storage
port (diagnosed to root cause via an `sb7` minimal repro).

Bind both as generalized schemes via `bind_scheme` (`poly1` helper:
fresh tyvar quantified in `sc_tyvars`, instantiated per call site), so
`len`/`show` are properly polymorphic.

Verified green prior to an environment loss (WSL /tmp wipe): the sb7
repro and the full ubicity storage.affine compiled; the 5 Deno-ESM
harnesses and `dune runtest` (214 tests, unchanged — same 2 pre-existing
E2E Node-CJS vscode failures) all passed. CI re-verifies the build here.

Refs #122 (follow-up to #123; unblocks ubicity#30).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: hyperpolymath

lib/typecheck.ml

@@ -1205,12 +1205,21 @@ let register_builtins (ctx : context) : unit =
   bind_var ctx "max" int_binop;
   bind_var ctx "min" int_binop;
   bind_var ctx "pow_float" float_binop;
-  (* `len` is polymorphic over String and [T] (issue #122 v2.5):
-     stdlib/string.affine calls `len` on both. Broadened from
-     Array[tv]->Int to 'a->Int (matches the interpreter's dynamic len;
-     the codegen lowers it to `.length`, valid for string and array). *)
-  bind_var ctx "len" (let tv = fresh_tyvar 0 in
-    TArrow (tv, QOmega, ty_int, EPure));
+  (* `len` / `show` are TRULY polymorphic — bound as generalized schemes
+     so each call site instantiates a fresh tyvar. v2.5/v2.6 mistakenly
+     bound them via bind_var as `'a -> _` with a SINGLE SHARED tyvar:
+     the first `len(String)` pinned it, then `len([T])` failed with
+     `Unify TypeMismatch (String, Array[T])` (diagnosed via the sb7
+     minimal repro; blocked the ubicity#30 storage port). issue #122. *)
+  let poly1 (body_of : ty -> ty) : scheme =
+    let tv = fresh_tyvar 0 in
+    let v = (match tv with
+      | TVar r -> (match !r with Unbound (v, _) -> v | _ -> assert false)
+      | _ -> assert false) in
+    { sc_tyvars = [(v, Types.KType)]; sc_effvars = []; sc_rowvars = [];
+      sc_body = body_of tv }
+  in
+  bind_scheme ctx "len" (poly1 (fun a -> TArrow (a, QOmega, ty_int, EPure)));
   (* Honest string/char primitives underpinning stdlib/string.affine
      (issue #122 v2.5). Concrete String/Char types; the Deno-ESM backend
      lowers each to a JS intrinsic. char ::= TCon "Char". *)
@@ -1232,8 +1241,8 @@ let register_builtins (ctx : context) : unit =
   bind_var ctx "parse_float"  (TArrow (ty_string, QOmega, opt ty_float, EPure));
   bind_var ctx "char_to_int"  (TArrow (ty_char, QOmega, ty_int, EPure));
   bind_var ctx "int_to_char"  (TArrow (ty_int, QOmega, ty_char, EPure));
-  bind_var ctx "show"
-    (let tv = fresh_tyvar 0 in TArrow (tv, QOmega, ty_string, EPure));
+  bind_scheme ctx "show"
+    (poly1 (fun a -> TArrow (a, QOmega, ty_string, EPure)));
   bind_var ctx "panic" (TArrow (ty_string, QOmega, ty_never, EPure));
   bind_var ctx "exit" (TArrow (ty_int, QOmega, ty_never, ESingleton "IO"));
   (* TEA runtime — accepts any record, returns unit with IO effect *)