refactor(application): filter_rules.py 更名 rules.py 并合并 RuleHelper

规则域收敛为单一事实来源 app/application/rules.py：
- filter_rules.py 更名为 rules.py（内置规则集 + RuleParser）
- filter.py 仅含 66 行 RuleHelper（用户规则组配置访问），并入 rules.py
- 同步更新 5 处引用方导入、nettest stub 目标与架构文档
- 兼容映射 app.helper.rule 指向新路径，legacy 导入仍可用

docs/rules/05-architecture.md

@@ -379,7 +379,7 @@ policy. `app/db` therefore has no dependency on `app/domain`.
 | `app/adapters/system/resource.py` | Runtime resource detection/download/installation |
 | `app/adapters/system/fsproxy.py` | Timeout-guarded local filesystem operations in a killable subprocess (with colocated `fsworker.py`) |
 | `app/adapters/external/wechat_crypt.py` | WeChat enterprise-message XML encryption/decryption protocol |
-| `app/application/filter_rules.py` | Built-in torrent filter rule set and rule parser |
+| `app/application/rules.py` | Rule domain: user rule-group config access (`RuleHelper`), built-in torrent filter rule set and rule parser |
 | `app/adapters/external/market.py` | Plugin repository discovery and installation |
 | `app/application/security/url.py` | URL/path validation, SSRF protection and signed image policy |
 | `app/application/mediaserver.py` | Configured media-server discovery and identity matching |