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/interface-migration-phase.php

@@ -0,0 +1,37 @@
+<?php
+/**
+ * Contract for a single migration phase (Strategy pattern).
+ *
+ * @package TMDO
+ * @since 3.0.1
+ */
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit;
+}
+
+/**
+ * Contract for a single migration phase (Strategy pattern).
+ *
+ * Each phase is idempotent — re-entering returns ok if the desired state is
+ * already achieved.  The orchestrator never calls execute() on a phase that
+ * has already returned 'ok'; it only re-enters on 'in_progress'.
+ */
+interface TMDO_Migration_Phase_Interface {
+
+	/** Short slug matching the PHASES constant entry (e.g. 'backfill_bulk'). */
+	public function name(): string;
+
+	/**
+	 * Run the phase.
+	 *
+	 * @param  array $job Job array passed by reference — phase may append log
+	 *                    lines and update metrics fields.
+	 * @return array{status: 'ok'|'in_progress'|'done', message?: string}
+	 *              'ok'          — phase finished, advance to next.
+	 *              'in_progress' — phase needs another tick (e.g. 24h window).
+	 *              'done'        — terminal phase completed (completed phase only).
+	 * @throws \RuntimeException On unrecoverable failure; orchestrator catches.
+	 */
+	public function execute( array &$job ): array;
+}