refactor: resolve_episode_url 阶梯构造拆解（58 行 / 圈 19 / 嵌套 3 → 36 行 / 圈 12 / 嵌套 2）

为什么：播客单集直链解析的唯一入口，前 20 行是降级阶梯的嵌套构造
（if 套 for 套 if），与尝试循环混在同一个函数里，改音质口径要通读全函数。

拆出：
- _falldown_order（模块级纯函数）：降级尝试序列——auto→通用阶梯原序 /
  已知音质→向下后缀 / 未知音质→该音质优先+通用阶梯，去重保序收敛为
  dict.fromkeys；_FALLDOWN_LADDER 从函数内上移模块级
- resolve_episode_url 只剩尝试循环；song_id 的 int 防御上提循环外
  （循环不变量：原实现首轮即判、失败直接 None，url_v1 从未收到过坏 sid）

先写测试后动代码：新增 tests/test_resolve_episode_url.py 11 例，在未改动
实现上一次跑绿，钉住核心口径——「只向下不向上」（standard 绝不回退到
exhigh，历史坑）、未知音质前置、垃圾 song_id 零请求、APIException 降级、
data[0] 无 url 降级、结果映射缺省值。
门禁：ruff 全过 · pyright 0/0 · pytest 370 passed（359→370）。

tests/test_resolve_episode_url.py

@@ -0,0 +1,121 @@
+# -*- coding: utf-8 -*-
+"""resolve_episode_url 的行为契约测试（沙箱，不触真实上游）。
+
+为什么：播客单集直链解析的唯一入口（音质降级阶梯 + 尝试循环），
+此前零直接测试。降级阶梯的口径是核心可观察行为：
+- auto → QUALITY_LADDER 原序
+- 显式音质 → 只向下不向上（standard 绝不回退到更高的 exhigh）
+- 未知音质 → 该音质优先 + 通用阶梯
+- song_id 垃圾值 → None 且不发任何请求
+
+替身边界：monkeypatch ``podcast.url_v1``（模块级名字，podcast.py 从
+.compat 导入后的绑定），按 level 返回结果或抛异常，并记录调用序列。
+"""
+import pytest
+
+from music_api import APIException
+from music_api import podcast as podcast_mod
+from music_api.podcast import resolve_episode_url
+
+
+@pytest.fixture()
+def url_calls(monkeypatch):
+    calls: list[tuple[int, str]] = []
+    state: dict = {'results': {}}
+
+    def _fake_url_v1(sid, level, cookies):
+        calls.append((sid, level))
+        r = state['results'].get(level)
+        if isinstance(r, Exception):
+            raise r
+        return r
+
+    monkeypatch.setattr(podcast_mod, 'url_v1', _fake_url_v1)
+    state['calls'] = calls
+    return state
+
+
+def _ok(level: str = 'standard') -> dict:
+    return {'data': [{'url': 'https://x/1.mp3', 'level': level, 'type': 'mp3',
+                      'size': '123', 'br': 128000, 'md5': 'abc'}]}
+
+
+async def test_auto_tries_quality_ladder_in_order(url_calls):
+    """auto：按 QUALITY_LADDER 原序尝试，全失败返回 None。"""
+    assert resolve_episode_url(101, {}, 'auto') is None
+    assert url_calls['calls'] == [(101, 'exhigh'), (101, 'standard')]
+
+
+async def test_explicit_level_falls_down_only(url_calls):
+    """显式 lossless：向后降级 exhigh/standard，绝不向上尝试 hires。"""
+    assert resolve_episode_url(101, {}, 'lossless') is None
+    assert url_calls['calls'] == [(101, 'lossless'), (101, 'exhigh'), (101, 'standard')]
+
+
+async def test_standard_never_falls_up(url_calls):
+    """显式 standard：只试一次（历史坑：原实现会回退到更高的 exhigh）。"""
+    assert resolve_episode_url(101, {}, 'standard') is None
+    assert url_calls['calls'] == [(101, 'standard')]
+
+
+async def test_unknown_level_prepends_generic_ladder(url_calls):
+    """未知音质：先试该音质，再退通用阶梯（去重保序）。"""
+    assert resolve_episode_url(101, {}, 'sky') is None
+    assert url_calls['calls'] == [(101, 'sky'), (101, 'exhigh'), (101, 'standard')]
+
+
+async def test_first_hit_returns_mapped_result(url_calls):
+    url_calls['results']['exhigh'] = _ok('hires')
+    out = resolve_episode_url(101, {}, 'auto')
+    assert out == {'url': 'https://x/1.mp3', 'level': 'hires', 'type': 'mp3',
+                   'size': 123, 'br': 128000, 'md5': 'abc', 'requested_level': 'exhigh'}
+
+
+async def test_level_defaults_to_requested_when_missing(url_calls):
+    url_calls['results']['exhigh'] = {'data': [{'url': 'https://x/1.mp3'}]}
+    out = resolve_episode_url(101, {}, 'auto')
+    assert out is not None
+    assert out['level'] == 'exhigh'
+    assert out['type'] == 'mp3'
+    assert out['md5'] == ''
+    assert out['size'] is None
+
+
+async def test_api_exception_tries_next_level(url_calls):
+    url_calls['results']['exhigh'] = APIException('无版权')
+    url_calls['results']['standard'] = _ok()
+    out = resolve_episode_url(101, {}, 'auto')
+    assert out is not None
+    assert out['requested_level'] == 'standard'
+    assert url_calls['calls'] == [(101, 'exhigh'), (101, 'standard')]
+
+
+async def test_missing_url_tries_next_level(url_calls):
+    """data[0] 无 url：视同失败，继续降级。"""
+    url_calls['results']['exhigh'] = {'data': [{'url': None}]}
+    url_calls['results']['standard'] = _ok()
+    out = resolve_episode_url(101, {}, 'auto')
+    assert out is not None
+    assert out['requested_level'] == 'standard'
+
+
+async def test_empty_data_tries_next_level(url_calls):
+    url_calls['results']['exhigh'] = None  # res 为 None → data 为 []
+    url_calls['results']['standard'] = _ok()
+    out = resolve_episode_url(101, {}, 'auto')
+    assert out is not None
+    assert out['requested_level'] == 'standard'
+
+
+async def test_bad_song_id_returns_none_without_requests(url_calls):
+    """song_id 垃圾值：None，且不发任何请求（防御原 TypeError 500）。"""
+    for bad in (None, 'abc', ''):
+        assert resolve_episode_url(bad, {}, 'auto') is None
+    assert url_calls['calls'] == []
+
+
+async def test_numeric_string_song_id_accepted(url_calls):
+    url_calls['results']['exhigh'] = _ok()
+    out = resolve_episode_url(' 101 ', {}, 'auto')
+    assert out is not None
+    assert url_calls['calls'][0] == (101, 'exhigh')