feat(form-builder): FormBindingApplicator + BindingActivityLogger (WS-6)

Orchestrates per-purpose subject resolution + binding conflict
resolution + per-binding writes per RFC Q4/Q7/Q9. Per-binding failures
captured in BindingPassResult, not thrown — partial failures are
expected and recoverable. Catastrophic failures (no transaction,
unknown purpose, missing schema) throw FormBindingApplicatorException
and bubble.

Per-strategy null-winner matrix implemented via a NO_OP sentinel:
overwrite=write null, append=noop, replace=conditional, first_write_wins=
write only into null target. Append is collection-only with set-merge
semantics (deduplicated array_merge).

Identity-key bindings are skipped during apply — the subject resolver
already used them for lookup/provisioning; re-writing is a no-op or a
clobber.

Activity log hierarchical: one bindings_pass_completed parent +
N binding_applied children with parent_activity_id linkage (RFC Q12).
Failed bindings get error_class/error_message in their activity entry
in addition to their FormSubmissionActionFailure row (deliberate
dual source of truth).

Refs: RFC-WS-6.md §3 (Q4, Q7, Q9, Q12)

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

api/app/Exceptions/FormBuilder/FormBindingApplicatorException.php

@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\FormBuilder;
+
+use RuntimeException;
+
+/**
+ * RFC-WS-6 §3 (Q3) — catastrophic applicator failure that bubbles to
+ * the caller. Per-binding failures are captured in BindingPassResult,
+ * not thrown.
+ */
+final class FormBindingApplicatorException extends RuntimeException
+{
+    public function __construct(
+        public readonly string $reasonCode,
+        public readonly string $submissionId,
+        ?string $message = null,
+    ) {
+        parent::__construct($message ?? "FormBindingApplicator failed: {$reasonCode} (submission {$submissionId})");
+    }
+}