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/transpile.sx

@@ -46,6 +46,7 @@
       ((= g "⍕") apl-quad-fmt)
       ((= g "⎕FMT") apl-quad-fmt)
       ((= g "⎕←") apl-quad-print)
+      ((= g "⍸") apl-where)
       (else (error "no monadic fn for glyph")))))
 
 (define
@@ -90,6 +91,7 @@
       ((= g "⍉") apl-transpose-dyadic)
       ((= g "⊢") (fn (a b) b))
       ((= g "⊣") (fn (a b) a))
+      ((= g "⍸") apl-interval-index)
       (else (error "no dyadic fn for glyph")))))
 
 (define