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.

docs/architecture/introduction.md

@@ -30,6 +30,7 @@ Each framework module is one Go package under `modules/`, documented by its READ
 | Admin | [cabana](../../modules/cabana/README.md), [boardwalk](../../modules/boardwalk/README.md) |
 | Services | [phrasebook](../../modules/phrasebook/README.md), [postcard](../../modules/postcard/README.md), [conga](../../modules/conga/README.md), [lighthouse](../../modules/lighthouse/README.md), [flare](../../modules/flare/README.md), [sunscreen](../../modules/sunscreen/README.md) |
 | Console and tooling | [bonfire](../../modules/bonfire/README.md), [tide](../../modules/tide/README.md) |
+| Utilities | [sieve](../../modules/sieve/README.md) |
 
 ## What runs where
 

docs/setup/coming-from-wintercms.md

@@ -23,6 +23,7 @@ Each row names the WinterCMS concept, the SummerCMS identifiers that replace it,
 | `registerPermissions`, `registerNavigation` | `pact.HasPermissions` returning `pact.Permission` values, `pact.HasNavigation` returning `pact.NavigationItem` values | [Users and permissions](../backend/users-and-permissions.md), [pact](../../modules/pact/README.md) |
 | `version.yaml` and the `updates/` directory | `pact.HasMigrations` returning an ordered gormigrate set; `lagoon.Migrate` runs every plugin's set and `lagoon.RollbackLast` undoes the last one | [Migrations](../database/migrations.md), [lagoon](../../modules/lagoon/README.md) |
 | Eloquent models | GORM structs with lagoon helpers: `lagoon.Fill` for mass assignment, `lagoon.Validate` for rules, `lagoon.Jsonable` for JSON columns, `lagoon.Page` for pagination | [Models](../database/models.md), [Casts and validation](../database/casts-and-validation.md), [lagoon](../../modules/lagoon/README.md) |
+| Laravel collections: `map`, `filter`, `reduce`, `keyBy`, `groupBy`, `unique`, `partition`, `first` and `pluck` | Generic functions on plain slices and maps: `sieve.Map`, `sieve.TryMap`, `sieve.Filter`, `sieve.Reduce`, `sieve.KeyBy`, `sieve.GroupBy`, `sieve.Uniq`, `sieve.Partition` and `sieve.Find`. Pluck is `sieve.Map` with a field accessor, unique by key is `sieve.UniqBy`, `chunk` is the standard `slices.Chunk`, and sets are built with `sieve.SetOf` | [sieve](../../modules/sieve/README.md) |
 | `fields.yaml` and `columns.yaml` | The same YAML, embedded in the plugin through `pact.AdminAssets` and compiled at boot into a `cabana.CompiledController` | [Forms](../backend/forms.md), [Lists and filters](../backend/lists-and-filters.md), [cabana](../../modules/cabana/README.md) |
 | Backend controllers with the Form, List and Relation behaviours | A `pact.AdminController` returned from `pact.HasAdminControllers`; the generic admin API replaces the behaviours, and hooks such as `pact.FormBeforeCreate` and `pact.ListExtendQuery` replace behaviour overrides | [Admin controllers](../backend/admin-controllers.md), [cabana](../../modules/cabana/README.md) |
 | `routes.php` | `pact.HasRoutes`, declaring routes on a `pact.Router` with groups, middleware names and `pact.Router.Where` constraints | [Routing](../services/routing.md), [surf](../../modules/surf/README.md) |
@@ -53,7 +54,7 @@ SummerCMS does not port the WinterCMS frontend or the PHP helpers that Go alread
 | The media manager | Not provided. Store uploads as model attachments; see [Attachments](../database/attachments.md). |
 | Import and export in backend lists | Not provided. |
 | Record sorting (the Reorder behaviour) | Not provided. |
-| Collections | Not provided. Use Go slices and the `slices` and `maps` packages. |
+| The fluent collection object (the Collection class and method chaining) | Not provided. Call the [sieve](../../modules/sieve/README.md) functions and the standard `slices` and `maps` packages on plain slices and maps. |
 | Behaviours and dynamic class extension | Not provided. Use Go interfaces and composition. |
 | Cache | Not provided. Use the Go standard library or a service another plugin publishes. |
 | Session | Not provided. The API is stateless and authenticates with tokens; see [Frontend and AJAX](../services/frontend-and-ajax.md). |