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.

docs/spec/03-special-forms.md

@@ -66,7 +66,8 @@ reference; it differs from Scheme.
 
 S1–S3, E1–E2 → jolt `forms-spec` "if/do/def" group; truthiness group in
 `truthiness-spec`; clojure-test-suite `core_test/if.cljc`. S4 → `forms-spec`
-fn/loop recur cases. X1 → UNVERIFIED (arity-error case to add).
+fn/loop recur cases. X1 → `forms-spec` "if arity (X1)" (0/1/4-arg forms throw
+in both the analyzer and the interpreter).
 
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