feat(form-builder): add BindingTypeRegistry as single source of truth for target shapes (WS-6)

Config-driven mapping from (target_entity, target_attribute) to storage
shape (scalar/collection/relation), PHP type, and identity-key
eligibility. Replaces any name-suffix matching (e.g. _tags, _skills) —
those are convention-not-contract and reject by design.

Used by publish guards now and (in session 2) by FormBindingApplicator.

Refs: RFC-WS-6.md §4 (V1)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/app/FormBuilder/Bindings/BindingTargetMeta.php

@@ -0,0 +1,21 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\FormBuilder\Bindings;
+
+use App\Enums\FormBuilder\BindingTargetType;
+
+/**
+ * RFC-WS-6 §4 (V1) — single config row from
+ * config/form_builder/binding_targets.php, materialised by
+ * {@see BindingTypeRegistry::resolve()}.
+ */
+final readonly class BindingTargetMeta
+{
+    public function __construct(
+        public BindingTargetType $type,
+        public string $php,
+        public bool $identityKeyEligible,
+    ) {}
+}

api/app/FormBuilder/Bindings/BindingTypeRegistry.php

@@ -0,0 +1,129 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\FormBuilder\Bindings;
+
+use App\Enums\FormBuilder\BindingTargetType;
+use App\Enums\FormBuilder\FormFieldBindingMergeStrategy;
+use App\Exceptions\FormBuilder\InvalidBindingTargetException;
+use App\Exceptions\FormBuilder\UnknownBindingTargetException;
+use Illuminate\Contracts\Config\Repository as ConfigRepository;
+
+/**
+ * RFC-WS-6 §4 (V1) — single source of truth for the storage shape of
+ * binding-target attributes. Consumed by publish guards
+ * (AppendStrategyRequiresCollectionTarget) and (in session 2) by
+ * FormBindingApplicator.
+ *
+ * NOT name-suffix matching (e.g. "ends with _tags"); convention-not-
+ * contract is rejected by design.
+ */
+final class BindingTypeRegistry
+{
+    /**
+     * @var array<string, array<string, BindingTargetMeta>>|null
+     */
+    private ?array $cache = null;
+
+    public function __construct(private readonly ConfigRepository $config) {}
+
+    /**
+     * @throws UnknownBindingTargetException
+     */
+    public function resolve(string $entity, string $attribute): BindingTargetMeta
+    {
+        $entries = $this->resolveTable();
+        if (! isset($entries[$entity][$attribute])) {
+            throw new UnknownBindingTargetException($entity, $attribute);
+        }
+
+        return $entries[$entity][$attribute];
+    }
+
+    public function isKnown(string $entity, string $attribute): bool
+    {
+        $entries = $this->resolveTable();
+
+        return isset($entries[$entity][$attribute]);
+    }
+
+    public function isIdentityKeyEligible(string $entity, string $attribute): bool
+    {
+        return $this->isKnown($entity, $attribute)
+            && $this->resolve($entity, $attribute)->identityKeyEligible;
+    }
+
+    /**
+     * @return list<string>
+     */
+    public function entities(): array
+    {
+        return array_keys($this->resolveTable());
+    }
+
+    /**
+     * @return list<string>
+     */
+    public function attributesFor(string $entity): array
+    {
+        $entries = $this->resolveTable();
+        if (! isset($entries[$entity])) {
+            return [];
+        }
+
+        return array_keys($entries[$entity]);
+    }
+
+    /**
+     * @throws UnknownBindingTargetException when (entity, attribute) is unknown
+     * @throws InvalidBindingTargetException when Append is paired with a non-COLLECTION target
+     */
+    public function validateAppendStrategy(
+        string $entity,
+        string $attribute,
+        FormFieldBindingMergeStrategy $strategy,
+    ): void {
+        if ($strategy !== FormFieldBindingMergeStrategy::Append) {
+            return;
+        }
+
+        $meta = $this->resolve($entity, $attribute);
+
+        if ($meta->type !== BindingTargetType::COLLECTION) {
+            throw new InvalidBindingTargetException(
+                entity: $entity,
+                attribute: $attribute,
+                reason: "merge_strategy=append requires a COLLECTION target; got {$meta->type->value}",
+            );
+        }
+    }
+
+    /**
+     * @return array<string, array<string, BindingTargetMeta>>
+     */
+    private function resolveTable(): array
+    {
+        if ($this->cache !== null) {
+            return $this->cache;
+        }
+
+        /** @var array<string, array<string, array{type: string, php: string, identity_key_eligible: bool}>> $raw */
+        $raw = $this->config->get('form_builder.binding_targets', []);
+
+        $built = [];
+        foreach ($raw as $entity => $attributes) {
+            foreach ($attributes as $attribute => $row) {
+                $built[$entity][$attribute] = new BindingTargetMeta(
+                    type: BindingTargetType::from($row['type']),
+                    php: $row['php'],
+                    identityKeyEligible: $row['identity_key_eligible'],
+                );
+            }
+        }
+
+        $this->cache = $built;
+
+        return $built;
+    }
+}