R7RS core: call/cc, raise/guard, multi-arity map, cond =>, do iteration

Phase 1 engine step 4 — R7RS compatibility primitives for the CEK evaluator.

call/cc: undelimited continuation capture with separate CallccContinuation
type (distinct from delimited shift/reset continuations). Escape semantics —
invoking k replaces the current continuation entirely.

raise/raise-continuable: proper CEK arg evaluation via raise-eval frame.
Non-continuable raise uses raise-guard frame that errors on handler return.
host-error primitive for safe unhandled exception fallback.

Multi-arity map: (map fn list1 list2 ...) zips multiple lists. Single-list
path unchanged for performance. New multi-map frame type.

cond =>: arrow clause syntax (cond (test => fn)) calls fn with test value.
New cond-arrow frame type.

R7RS do: shape-detecting dispatch — (do ((var init step) ...) (test result) body)
desugars to named let. Existing (do expr1 expr2) sequential form preserved.

integer? primitive, host-error alias. Transpiler fixes: match/case routing,
wildcard _ support, nested match arm handling.

2522/2524 OCaml tests pass (2 pre-existing scope failures from transpiler
match codegen, not related to these changes).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/lib/sx_runtime.ml

@@ -50,6 +50,8 @@ let sx_call f args =
     Thunk (l.l_body, local)
   | Continuation (k, _) ->
     k (match args with x :: _ -> x | [] -> Nil)
+  | CallccContinuation _ ->
+    raise (Eval_error "callcc continuations must be invoked through the CEK machine")
   | _ ->
     let nargs = List.length args in
     let args_preview = if nargs = 0 then "" else
@@ -103,6 +105,7 @@ let get_val container key =
      | "update-fn" -> f.cf_extra | "head-name" -> f.cf_extra
      | "emitted" -> f.cf_extra2 | "effect-list" -> f.cf_extra2
      | "first-render" -> f.cf_extra2 | "file" -> f.cf_env
+     | "extra" -> f.cf_extra | "extra2" -> f.cf_extra2
      | _ -> Nil)
   | Dict d, String k -> dict_get d k
   | Dict d, Keyword k -> dict_get d k
@@ -323,7 +326,20 @@ let continuation_data v = match v with
   | Continuation (_, None) -> Dict (Hashtbl.create 0)
   | _ -> raise (Eval_error "not a continuation")
 
+(* Callcc (undelimited) continuation support *)
+let callcc_continuation_p v = match v with CallccContinuation _ -> Bool true | _ -> Bool false
+
+let make_callcc_continuation captured =
+  CallccContinuation (sx_to_list captured)
+
+let callcc_continuation_data v = match v with
+  | CallccContinuation frames -> List frames
+  | _ -> raise (Eval_error "not a callcc continuation")
+
 (* Dynamic wind — simplified for OCaml (no async) *)
+let host_error msg =
+  raise (Eval_error (value_to_str msg))
+
 let dynamic_wind_call before body after _env =
   ignore (sx_call before []);
   let result = sx_call body [] in