Replace sequence_merge O(log N) recursion with O(1) fold expression

Use operator| with fold expression (Seqs{} | ...) to merge sequences
in O(1) template depth instead of O(log N) binary tree recursion.

- Reduces sequence_merge instantiations from 449 to 167 (63% reduction)
- Total template instantiations: 47,186 → 46,974 (-212)
- ADL finds operator| since Sequence is in ck namespace

Author: tenpercent

include/ck/utility/sequence.hpp

@@ -199,57 +199,20 @@ template <index_t N>
 using make_index_sequence =
     typename __make_integer_seq<impl::__integer_sequence, index_t, N>::seq_type;
 
-// merge sequence - optimized to avoid recursive instantiation
-namespace detail {
-
-// Helper to concatenate multiple sequences in one step using fold expression
-template <typename... Seqs>
-struct sequence_merge_impl;
-
-// Base case: single sequence
-template <index_t... Is>
-struct sequence_merge_impl<Sequence<Is...>>
-{
-    using type = Sequence<Is...>;
-};
-
-// Two sequences: direct concatenation
-template <index_t... Xs, index_t... Ys>
-struct sequence_merge_impl<Sequence<Xs...>, Sequence<Ys...>>
-{
-    using type = Sequence<Xs..., Ys...>;
-};
-
-// Three sequences: direct concatenation (avoids one level of recursion)
-template <index_t... Xs, index_t... Ys, index_t... Zs>
-struct sequence_merge_impl<Sequence<Xs...>, Sequence<Ys...>, Sequence<Zs...>>
-{
-    using type = Sequence<Xs..., Ys..., Zs...>;
-};
-
-// Four sequences: direct concatenation
-template <index_t... As, index_t... Bs, index_t... Cs, index_t... Ds>
-struct sequence_merge_impl<Sequence<As...>, Sequence<Bs...>, Sequence<Cs...>, Sequence<Ds...>>
+// merge sequence - O(1) template depth using fold expression
+// Binary merge operator for fold expression - enables O(1) depth via (S1 | S2 | S3 | ...)
+// Must be in ck namespace for ADL to find it when used with Sequence types
+template <index_t... As, index_t... Bs>
+constexpr Sequence<As..., Bs...> operator|(Sequence<As...>, Sequence<Bs...>)
 {
-    using type = Sequence<As..., Bs..., Cs..., Ds...>;
-};
-
-// General case: binary tree reduction (O(log N) depth instead of O(N))
-template <typename S1, typename S2, typename S3, typename S4, typename... Rest>
-struct sequence_merge_impl<S1, S2, S3, S4, Rest...>
-{
-    // Merge pairs first, then recurse
-    using left  = typename sequence_merge_impl<S1, S2>::type;
-    using right = typename sequence_merge_impl<S3, S4, Rest...>::type;
-    using type  = typename sequence_merge_impl<left, right>::type;
-};
-
-} // namespace detail
+    return {};
+}
 
 template <typename... Seqs>
 struct sequence_merge
 {
-    using type = typename detail::sequence_merge_impl<Seqs...>::type;
+    // Left fold: ((S1 | S2) | S3) | ... - O(1) template depth
+    using type = decltype((Seqs{} | ...));
 };
 
 template <>