core: variadic bit ops, set? covers sorted sets, if rejects extra forms

Three canonical-conformance fixes from the post-shrink batch:

- bit-and/bit-or/bit-xor/bit-and-not get Clojure's variadic arities as
  20-coll shells folding the binary host ops (now __bit-* seams). 2-arg call
  sites still compile to the native janet op via the backend's native-ops
  table. The passes.clj constant-fold table now names the seams — the public
  fns are overlay and don't exist when the compiler loads (this briefly broke
  every compile-mode init).

- core-set? recognizes the :jolt/sorted-set representation (jolt-dpn):
  (set? (sorted-set 1)) was false, and ifn? on sorted sets inherited the bug.

- (if) / (if test) / (if test then else extra) throw in both the analyzer
  and the interpreter — spec 03-special-forms X1, now marked verified.

Suite 4704 -> 4706; bench and the greeter example benchmark are flat.

test/spec/forms-spec.janet

@@ -81,3 +81,12 @@
   ["call literal fn" "1"      "((fn* [] 1))"]
   ["call nested"     "6"      "(+ ((fn* [] 1)) ((fn* [] 2)) ((fn* [] 3)))"]
   ["call nil"        :throws  "(nil)"])
+
+# if takes two or three argument forms — anything else is a compile-time
+# error (spec 03-special-forms X1).
+(defspec "forms / if arity (X1)"
+  ["bare if throws"      :throws "(if)"]
+  ["one-arg if throws"   :throws "(if true)"]
+  ["four-arg if throws"  :throws "(if true 1 2 3)"]
+  ["two-arg if ok"       "nil"   "(if false 1)"]
+  ["three-arg if ok"     "2"     "(if false 1 2)"])

test/spec/numbers-spec.janet

@@ -204,3 +204,17 @@
   ["== int and double"      "true"  "(== 1 1.0)"]
   ["== throws on non-number" :throws "(== 1 :a)"]
   ["== throws on two keywords" :throws "(== :a :a)"])
+
+# Variadic bit ops (canonical Clojure arities: binary host op folded over the
+# rest). Shift/clear/set/flip/test stay 2-arg, as in Clojure.
+(defspec "numbers / variadic bit ops"
+  ["bit-and 2"        "4"  "(bit-and 12 6)"]
+  ["bit-and 3"        "4"  "(bit-and 12 6 7)"]
+  ["bit-and 4"        "0"  "(bit-and 12 6 7 3)"]
+  ["bit-or 3"         "7"  "(bit-or 1 2 4)"]
+  ["bit-xor 3"        "7"  "(bit-xor 1 2 4)"]
+  ["bit-xor folds left" "1" "(bit-xor 5 6 2)"]
+  ["bit-and-not 2"    "8"  "(bit-and-not 12 6)"]
+  ["bit-and-not 3"    "8"  "(bit-and-not 12 6 3)"]
+  ["bit-and single arg throws" :throws "(bit-and 5)"]
+  ["bit-or keeps binary fast path" "3" "(bit-or 1 2)"])

test/spec/sets-spec.janet

@@ -43,3 +43,16 @@
   ["join"                   "#{{:a 1, :b 2, :c 3}}" "(do (require (quote [clojure.set :as s])) (s/join #{{:a 1 :b 2}} #{{:b 2 :c 3}}))"]
   ["map-invert"             "{1 :a}"     "(do (require (quote [clojure.set :as s])) (s/map-invert {:a 1}))"]
   ["rename-keys"            "{:b 1}"     "(do (require (quote [clojure.set :as s])) (s/rename-keys {:a 1} {:a :b}))"])
+
+# set? recognizes every set representation (jolt-dpn: sorted sets are tagged
+# tables the host set? predicate missed).
+(defspec "set / set? across representations"
+  ["literal"        "true"  "(set? #{1})"]
+  ["empty literal"  "true"  "(set? #{})"]
+  ["sorted-set"     "true"  "(set? (sorted-set 1 2))"]
+  ["sorted-set-by"  "true"  "(set? (sorted-set-by > 1 2))"]
+  ["empty sorted"   "true"  "(set? (sorted-set))"]
+  ["map is not"     "false" "(set? {})"]
+  ["vector is not"  "false" "(set? [1])"]
+  ["coll? still true" "true" "(coll? (sorted-set 1))"]
+  ["ifn? sorted-set" "true" "(ifn? (sorted-set 1))"])