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/account/handlers.rs

@@ -214,7 +214,7 @@ pub async fn dashboard_stats(
 // ============ Devices ============
 
 #[derive(serde::Deserialize)]
-pub(super) struct RegisterDeviceRequest {
+pub struct RegisterDeviceRequest {
     #[serde(default)]
     device_id: String,
     #[serde(default)]