Fold type predicates on proven types (jolt-wcw) (#129)

When the collection-type inference proves an argument's type, number?/
string?/keyword?/record?/nil?/some? fold to a compile-time boolean. A
const-fold now runs after inference so a folded predicate propagates and
collapses any if it gates to the taken branch.

Sound by construction: only a provable answer folds, and only when the
argument is side-effect-free (a const or local) so dropping its evaluation
is a no-op. Unknown types (:any/:truthy) and impure args keep the call.
vector?/set?/map? are left out — the :vec tag conflates a real vector with
a range/seq, so vector? could be wrong.

50M-iter loop, same shape isolated with a carry-only control: number? call+
branch 5080ms, predicate folded 1365ms — matching the 1417ms control floor,
so the 3.7x is entirely the eliminated call+branch.

Co-authored-by: Yogthos <yogthos@gmail.com>

test/integration/predicate-fold-test.janet

@@ -0,0 +1,61 @@
+# Predicate folding from inference (jolt-wcw): when the collection-type
+# inference PROVES the argument's type, a type predicate (number?/string?/
+# keyword?/nil?/some?/record?) folds to a compile-time boolean constant, which
+# the trailing const-fold then propagates — collapsing any `if` it gates to the
+# taken branch. Sound: only a provable answer folds, and only when the argument
+# is side-effect-free (a local or const), so dropping its evaluation is a no-op.
+# Mirrors type-infer-test.janet's harness (count a marker in the emitted IR to
+# prove the optimization fired, then evaluate to prove it stayed correct).
+(import ../../src/jolt/api :as api)
+(import ../../src/jolt/backend :as backend)
+(import ../../src/jolt/reader :as reader)
+
+(print "Predicate folding (jolt-wcw)...")
+
+(os/setenv "JOLT_DIRECT_LINK" "1")
+(def ctx (api/init {:compile? true}))
+(api/eval-string ctx "(ns pf)")
+
+(defn code [src]
+  (string/format "%p" (backend/emit-ir ctx (backend/analyze-form ctx (reader/parse-string src)))))
+(defn occurs [src needle] (length (string/find-all needle (code src))))
+(defn ev [src] (api/eval-string ctx src))
+
+# --- the predicate call is gone where the type is proven --------------------
+(assert (= 0 (occurs "(fn [] (let [x (+ 1 2)] (number? x)))" "number?"))
+        "number? on proven :num -> folded, call eliminated")
+(assert (= 0 (occurs "(fn [] (let [x \"hi\"] (string? x)))" "string?"))
+        "string? on proven :str -> folded")
+(assert (= 0 (occurs "(fn [] (let [x :k] (keyword? x)))" "keyword?"))
+        "keyword? on proven :kw -> folded")
+(assert (= 0 (occurs "(fn [] (let [x (+ 1 2)] (nil? x)))" "nil?"))
+        "nil? on a provably non-nil value -> folded")
+
+# --- the folded constant collapses an if it gates --------------------------
+# the dead branch's literal (200) must be gone after dead-branch removal
+(assert (= 0 (occurs "(fn [] (let [x (+ 1 2)] (if (number? x) 100 200)))" "200"))
+        "true predicate folds the if to its then-branch (dead 200 dropped)")
+(assert (= 0 (occurs "(fn [] (let [x (+ 1 2)] (if (string? x) 100 200)))" "100"))
+        "false predicate folds the if to its else-branch (dead 100 dropped)")
+
+# --- sound fallback: unknown type or impure arg keeps the call -------------
+# a param is :any (Phase 0 doesn't type it) -> no fold
+(assert (= 1 (occurs "(fn [m] (number? m))" "number?"))
+        "unknown-type arg keeps the predicate call")
+# arg type is proven :num but the arg has side effects (a call) -> must NOT
+# drop its evaluation, so the predicate is left in place
+(assert (>= (occurs "(fn [g] (number? (+ (g) 1)))" "number?") 1)
+        "impure arg (even with proven type) keeps the predicate call")
+
+# --- correctness: folded path evaluates to the dispatched path -------------
+(assert (= true  (ev "((fn [] (let [x (+ 1 2)] (number? x))))")) "number? true value")
+(assert (= false (ev "((fn [] (let [x \"hi\"] (number? x))))")) "number? false value")
+(assert (= true  (ev "((fn [] (let [x :k] (keyword? x))))")) "keyword? true value")
+(assert (= false (ev "((fn [] (let [x 5] (nil? x))))")) "nil? false value")
+(assert (= true  (ev "((fn [] (let [x 5] (some? x))))")) "some? true value")
+(assert (= :yes  (ev "((fn [] (let [x 5] (if (number? x) :yes :no))))")) "gated if takes proven branch")
+(assert (= :no   (ev "((fn [] (let [x 5] (if (string? x) :yes :no))))")) "gated if drops false branch")
+# impure arg still runs its side effect and returns the right answer
+(assert (= 6 (ev "((fn [g] (if (number? (+ (g) 1)) (+ (g) 5) 0)) (fn [] 1))")) "impure-arg predicate stays correct")
+
+(print "Predicate folding (jolt-wcw) passed!")