feat(sieve): add the remaining collection helpers and docs mentions

- Add TryMap, Filter, Reduce, KeyBy, GroupBy, Uniq, UniqBy, Partition
  and Find, plus the Set type built with SetOf (Has, Add, Slice).
- Add unit tests at 100% statement coverage and a runnable example for
  every function and Set method.
- Complete the module README: contracts, use cases, Pluck rationale,
  standard-library pointers and the full API reference.
- docs/architecture/introduction.md: add a Utilities row for sieve.
- docs/setup/coming-from-wintercms.md: map Laravel collection methods
  to sieve and narrow the not-provided row to the fluent collection
  object.

modules/sieve/set.go

@@ -0,0 +1,43 @@
+package sieve
+
+// Set is a set of comparable values backed by a plain map, so len, range,
+// delete and maps.Keys work on it directly. Create sets with SetOf: Has and
+// Slice work on a nil Set, but Add on a nil Set panics like any write to a nil
+// map. Slice returns the elements in unspecified order; use
+// slices.Sorted(maps.Keys(set)) for ordered output when T is ordered.
+type Set[T comparable] map[T]struct{}
+
+// SetOf returns a new set holding items, without duplicates. SetOf() with no
+// arguments returns an empty, non-nil set that Add can use. It accepts a slice
+// spread with ..., including a named slice type.
+func SetOf[T comparable](items ...T) Set[T] {
+	s := make(Set[T], len(items))
+	for _, v := range items {
+		s[v] = struct{}{}
+	}
+	return s
+}
+
+// Has reports whether v is in the set. A nil Set has no elements.
+func (s Set[T]) Has(v T) bool {
+	_, ok := s[v]
+	return ok
+}
+
+// Add puts items into the set; items already present are ignored. Add on a nil
+// Set panics, so create sets with SetOf.
+func (s Set[T]) Add(items ...T) {
+	for _, v := range items {
+		s[v] = struct{}{}
+	}
+}
+
+// Slice returns a new slice holding each element of the set exactly once, in
+// unspecified order. It is non-nil even for an empty or nil Set.
+func (s Set[T]) Slice() []T {
+	out := make([]T, 0, len(s))
+	for v := range s {
+		out = append(out, v)
+	}
+	return out
+}