refactor(migration): 回填 Migration Phase Strategy 體系（PR-D）

A v3.0.1 把 orchestrator 的 11 個 phase 抽成可注入的 Phase 物件，B 仍是
1107 行單體、以 'phase_' . $current 字串魔法分派、completed 甚至 inline
在 tick() 裡。本 commit 對齊：

新增 13 檔
- includes/migration/interface-migration-phase.php
- includes/migration/class-tmdo-migration-phase-base.php（log/get_managed_keys/
  execute_bulk_pivot/values_loose_equal 等共用 helper）
- includes/migration/phases/ 11 個 phase 類別

orchestrator 1107 → 640 行
- tick() 改 make_phase() 工廠 + $phase->execute($job)
- 移除 final、self::ENTITY_TYPE → static::（A v3.3.0 late static binding）
- 保留 B 原有的 '✓ %s (%.2fs)' 耗時 log（改為 tick 自行量測，A 版已簡化掉）
- 公開介面（preflight/start/tick/get_status/cancel/resume/needs_attention/
  cron_tick）經 diff 確認與 A 完全一致，呼叫端零影響

連帶
- Schema_Manager 補 table_exists 的 request-scoped cache 與
  flush_table_exists_cache()（A v3.1.6 + v3.4.6），Phase 測試需要它
- back-compat 補 12 個 Phase 類別的 WPDO_ alias
- 移植 MigrationPhaseTest + MigrationPhaseRemainingTest（527 行）

測試隔離差異（B 的 unit bootstrap 會載入 Member_Fields / Post_Fields，
A 的不會）：MigrationPhaseRemainingTest 的 setUp 需額外清空 Entity_Registry
與 wpdo_register_entity_fields listener，否則 install_schema 會真的走進
dbDelta。MigrationPhaseTest 的 interface 斷言改用 TMDO_ 正式名稱
（PHP 無法 class_alias 介面，且該契約是核心內部擴充點）。

unit 451 / integration 398 GREEN

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TbG1keQQ7XBa7qMQY16KCY

includes/migration/phases/class-tmdo-phase-backfill-unserialize.php

@@ -0,0 +1,69 @@
+<?php
+/**
+ * Migration phase: row-by-row backfill for groups with JSON/serialized fields.
+ *
+ * @package TMDO
+ * @since 3.0.1
+ */
+
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+/**
+ * Uses TMDO_Entity_Migration_Engine for groups whose fields contain JSON type,
+ * which cannot be bulk-pivoted via INSERT … SELECT.
+ */
+class TMDO_Phase_Backfill_Unserialize extends TMDO_Migration_Phase_Base {
+
+	/**
+	 * Returns the phase slug.
+	 *
+	 * @return string
+	 */
+	public function name(): string {
+		return 'backfill_unserialize';
+	}
+
+	/**
+	 * Executes the phase.
+	 *
+	 * @param array $job Job array (by reference).
+	 * @return array{status: string}
+	 * @throws \RuntimeException On migration engine failure.
+	 */
+	public function execute( array &$job ): array {
+		if ( ! empty( $job['options']['dry_run'] ) ) {
+			$this->log( $job, '↪ Backfill unserialize skipped (dry_run)' );
+			return array( 'status' => 'ok' );
+		}
+
+		$total_rows = 0;
+
+		foreach ( TMDO_Entity_Registry::get_groups_for_type( $this->entity_type ) as $group ) {
+			if ( ! $this->group_has_json_field( $group ) ) {
+				continue; // Handled in backfill_bulk.
+			}
+
+			$result = TMDO_Entity_Migration_Engine::migrate_group(
+				$this->entity_type,
+				$group,
+				array( 'sleep_ms' => 0 )
+			);
+
+			if ( ! empty( $result['error'] ) ) {
+				throw new \RuntimeException( "Row-by-row backfill {$group} failed: " . $result['error'] );
+			}
+			if ( ( $result['errors'] ?? 0 ) > 0 && 0 === ( $result['migrated'] ?? 0 ) ) {
+				throw new \RuntimeException( "All rows failed during {$group} backfill — see error_log" );
+			}
+
+			$total_rows += (int) ( $result['migrated'] ?? 0 );
+			$this->log( $job, sprintf( '  • row-by-row %s: %d row(s)', $group, $result['migrated'] ?? 0 ) );
+		}
+
+		$this->log( $job, sprintf( 'Row-by-row backfill: %d rows total', $total_rows ) );
+		return array( 'status' => 'ok' );
+	}
+}