apl: ⍸ where — monadic indices-of-truthy + dyadic interval-index

- apl-where (monadic): ravel scan, filter non-zero, +⎕IO offsets
- apl-interval-index (dyadic): count of breaks ≤ y; broadcasts over
  scalar or vector Y
- Wired into apl-monadic-fn / apl-dyadic-fn cond chains
- +10 pipeline tests covering both arities, ⎕IO offsetting, edge cases
  (empty mask, all-truthy, y below/above all breaks)
- pipeline 109/109

lib/apl/runtime.sx

@@ -827,6 +827,31 @@
               ((new-ravel (reduce (fn (acc r) (append acc (map (fn (j) (nth ravel (+ (* r cols) j))) (range 0 cols)))) (list) kept-rows)))
               (make-array (cons (len kept-rows) (rest shape)) new-ravel))))))))
 
+(define
+  apl-where
+  (fn
+    (arr)
+    (let
+      ((ravel (get arr :ravel)) (io (disclose (apl-quad-io))))
+      (let
+        ((indices (filter (fn (i) (not (= (nth ravel i) 0))) (range 0 (len ravel)))))
+        (apl-vector (map (fn (i) (+ i io)) indices))))))
+
+(define
+  apl-interval-index
+  (fn
+    (breaks vals)
+    (let
+      ((b-ravel (get breaks :ravel))
+        (v-ravel
+          (if (scalar? vals) (list (disclose vals)) (get vals :ravel))))
+      (let
+        ((result (map (fn (y) (len (filter (fn (b) (<= b y)) b-ravel))) v-ravel)))
+        (if
+          (scalar? vals)
+          (apl-scalar (first result))
+          (make-array (get vals :shape) result))))))
+
 (define
   apl-primes
   (fn