fix(saas): eliminate 6 compiler warnings + stabilize directive complete

- Remove unused imports: Utc (billing/service), StatusCode (billing/handlers), Sha256 (billing/handlers)
- Fix unused variables: _db (scheduler), _e (payment WeChat error)
- Fix visibility: RegisterDeviceRequest pub(super) → pub (used in pub handler)
- Update STABILIZATION_DIRECTIVE.md: all 7 criteria met, downgrade to advisory
- Fix TRUTH.md §2.2: mark P0/P1 defects as resolved, update Admin pages count to 14

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/zclaw-saas/src/billing/payment.rs

@@ -465,7 +465,7 @@ async fn generate_wechat_url(
     }
 
     let resp_json: serde_json::Value = resp.json().await
-        .map_err(|e| SaasError::Internal("微信支付响应解析失败".into()))?;
+        .map_err(|_| SaasError::Internal("微信支付响应解析失败".into()))?;
 
     let code_url = resp_json.get("code_url")
         .and_then(|v| v.as_str())