feat(data_store): implement V3 cascading lookup and ID Vision standardization

- Added GET /v3/data_store/code/{code} with hierarchical context-aware fallback.
- Implemented ID Vision standard in Data_Store_Base (string IDs, internal int exclusion).
- Enhanced Data_Store_Base robustness to handle stringified 'NULL' values from the database.
- Fixed legacy router bugs by removing undefined parameters (inc_event_cfg, inc_event_location).
- Corrected type hints and resolved UnboundLocalError in data_store methods.
- Updated Frontend Integration Guide with Section 8: Data Store V3.
- Added unified E2E test script: tests/e2e/test_e2e_v3_data_store_lookup.py.

documentation/GUIDE__V3_FRONTEND_API.md

@@ -136,4 +136,35 @@ V3 returns machine-readable error objects in `meta.details` for failures.
 - `database_duplicate`: Non-unique value (Code 1062).
 - `database_constraint`: Foreign key violation (Codes 1451, 1452).
 - `database_schema`: Invalid column name (Codes 1054, 1146).
-- `validation`: Pydantic validation failed (Check `details` for field-specific errors).
+- `validation`: Pydantic validation failed (Check `details` for field-specific errors).
+
+---
+
+## 8. Data Store Cascading Lookup (V3)
+
+V3 provides a specialized endpoint for retrieving configuration or content snippets by a human-friendly `code`. This uses a **hierarchical fallback** logic to find the best fit for the current context.
+
+### A. The V3 Lookup Endpoint
+**Path:** `GET /v3/data_store/code/{code}`
+
+**Query Parameters:**
+| Parameter | Type | Required | Description |
+| :--- | :--- | :--- | :--- |
+| `for_type` | String | No | Parent object type (e.g., `event`, `person`). |
+| `for_id` | String | No | Parent object random ID. |
+
+### B. Cascading Logic (Specificity)
+The API automatically resolves the "best fit" record in the following order:
+1. **Record Specific:** Matches `for_type` + `for_id` (and account).
+2. **Account Specific:** Matches the `x-account-id` header.
+3. **Global Default:** Matches `code` where `account_id` is `NULL`.
+
+### C. Example Implementation
+```ts
+// GET /v3/data_store/code/event_launcher_main_info?for_type=event&for_id=nmBfuGFeR0k
+export async function get_data_store_v3({ api_cfg, code, for_type, for_id }) {
+    const endpoint = `/v3/data_store/code/${code}`;
+    const params = { for_type, for_id };
+    return await get_object({ api_cfg, endpoint, params });
+}
+```