docs: align v0.2 docs with element-owned filter architecture (PR #20)

Rewrites the developer documentation for the v0.2 filter subsystem rework:

- Delete pages for removed subsystems: DCA callbacks (callbacks.mdx),
  filter invokers (spec/invoker.md), and the six removed filter-element
  contracts (form-data, form-type-options, hydrate-form, intrinsic-value,
  runtime-value, palette).
- New pages: dev/filter-types (custom filter types guide), dev/dca-builder
  (backend DCA building via DcaContract/DcaBuilder/ElementDcaEvent),
  dev/contracts/dca-contract, spec/filtering (filter pipeline architecture),
  reference/filter-types (built-in type catalog), migrating-from-v0.1,
  removed-in-v0.2, and an unlisted preflight-v0.2 release checklist.
- Rewrite dev/filter-elements around the buildForm/buildFilter/buildDca/
  configureOptions lifecycle, dev/events for the new event set and named
  aliases (.building/.built/.form_built/.dca), spec/specifications around
  the immutable Filter value object and the keyed addFilter() API, and
  dev/contracts/index for the new contract layout.
- Update attribute docs (#[AsFilterElement] without palette/formType/method,
  #[AsListType] without palette), engine-mods examples to
  SimpleEquationFilterElement::define() + addFilter(), Twig docs for the
  new flare_make_filter() helper, and reference tables for the new
  Filter\Element namespace and per-element form controls.
- flare_enclosure_files is documented as removed (code removal tracked on
  the preflight page).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: ericges

docs/dev/callbacks.mdx

@@ -27,14 +27,15 @@ public function configureOptions(OptionsResolver $resolver): void
 
 ## Important distinction
 
-This contract is not the same as `FormTypeOptionsContract`.
+This contract is not the same as `FilterElementOptionsInterface`.
 
-- Use `OptionsInterface` to define and validate custom option structures with `OptionsResolver`.
-- Use `FormTypeOptionsContract` to build Symfony form field options for a rendered filter field.
+- Use `Filter\Element\FilterElementOptionsInterface` for a filter element's canonical config schema — it is
+  resolved by FLARE's filter pipeline and pairs `configureOptions()` with `configFromRow()`.
+- `Contract\OptionsInterface` is a generic marker for any service that wants to expose an
+  `OptionsResolver`-based option schema.
 
 ## Current state in this repository
 
-`AbstractFilterElement` implements this interface with a no-op method, and `SimpleEquationElement` overrides it.
-
-At the moment, there is no internal runtime call site in `src/` that invokes `OptionsInterface::configureOptions()`
-directly. Treat it as part of Flare's extensibility surface for code that wants to resolve custom options explicitly.
+At the moment, there is no implementor and no internal runtime call site in `src/` that invokes
+`OptionsInterface::configureOptions()` directly. Treat it as part of Flare's extensibility surface for code
+that wants to resolve custom options explicitly.

docs/dev/contracts/_options-interface.md

@@ -0,0 +1,40 @@
+# DcaContract
+
+The `DcaContract` lets a filter element or list type assemble its backend DCA configuration — palette and
+field tweaks — at runtime.
+
+**Interface:** `HeimrichHannot\FlareBundle\Contract\DcaContract`
+
+## Method
+
+### `buildDca(DcaBuilder $dca, DcaContext $context): void`
+
+Declare the type's palette and per-field adjustments on the `DcaBuilder`. The `DcaContext` provides the
+record being edited (`listModel`, `filterModel`) and lazy access to the list's execution context (table
+aliases, target table).
+
+FLARE's `loadDataContainer` listener applies the collected configuration to `$GLOBALS['TL_DCA']` for
+`tl_flare_filter` and `tl_flare_list`. After `buildDca()` runs, an
+[`ElementDcaEvent`](../dca-builder.md#5-modifying-another-types-dca) is dispatched so third parties can
+adjust the result.
+
+`AbstractFilterElement` implements this contract with a no-op default; list types opt in by implementing the
+interface.
+
+## Example
+
+```php
+use HeimrichHannot\FlareBundle\DataContainer\Builder\DcaBuilder;
+use HeimrichHannot\FlareBundle\DataContainer\Builder\DcaContext;
+
+public function buildDca(DcaBuilder $dca, DcaContext $context): void
+{
+    $dca->palette('{filter_legend},fieldGeneric,label');
+
+    $dca->field('fieldGeneric')
+        ->options(fn (): array => $this->getFieldOptions($context->getTargetTable()));
+}
+```
+
+See the [Backend DCA Building guide](../dca-builder.md) for the full `DcaBuilder`, `DcaFieldBuilder`, and
+`DcaContext` API.