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/Exceptions/FormBuilder/InvalidBindingTargetException.php

@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+use RuntimeException;
+
+/**
+ * RFC-WS-6 §4 (V1) — thrown when a binding configuration is structurally
+ * invalid for its target shape. Most common case: `merge_strategy=Append`
+ * on a SCALAR target.
+ */
+final class InvalidBindingTargetException extends RuntimeException
+{
+    public function __construct(
+        public readonly string $entity,
+        public readonly string $attribute,
+        public readonly string $reason,
+    ) {
+        parent::__construct(
+            "Invalid binding target {$entity}.{$attribute}: {$reason}",
+        );
+    }
+}