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_primitives.ml

@@ -218,6 +218,8 @@ let () =
     match args with [a] -> Bool (is_nil a) | _ -> raise (Eval_error "nil?: 1 arg"));
   register "number?" (fun args ->
     match args with [Number _] -> Bool true | [_] -> Bool false | _ -> raise (Eval_error "number?: 1 arg"));
+  register "integer?" (fun args ->
+    match args with [Number f] -> Bool (Float.is_integer f) | [_] -> Bool false | _ -> raise (Eval_error "integer?: 1 arg"));
   register "string?" (fun args ->
     match args with [String _] -> Bool true | [_] -> Bool false | _ -> raise (Eval_error "string?: 1 arg"));
   register "boolean?" (fun args ->
@@ -708,6 +710,10 @@ let () =
     match args with [String msg] -> raise (Eval_error msg)
     | [a] -> raise (Eval_error (to_string a))
     | _ -> raise (Eval_error "error: 1 arg"));
+  register "host-error" (fun args ->
+    match args with [String msg] -> raise (Eval_error msg)
+    | [a] -> raise (Eval_error (to_string a))
+    | _ -> raise (Eval_error "host-error: 1 arg"));
   register "try-catch" (fun args ->
     match args with
     | [try_fn; catch_fn] ->