Update docs.

Author: LZL0

ADVANCED.md

@@ -1067,7 +1067,7 @@ result = await l0.run(
 ### Fallback Events
 
 ```python
-{"type": "FALLBACK_START", "ts": ..., "from_model": ..., "to_model": ..., "reason": ...}
+{"type": "FALLBACK_START", "ts": ..., "fromIndex": ..., "toIndex": ..., "reason": ...}
 {"type": "FALLBACK_MODEL_SELECTED", "ts": ..., "index": ..., "model": ...}
 {"type": "FALLBACK_END", "ts": ..., "index": ..., "duration_ms": ...}
 ```

API.md

@@ -471,7 +471,7 @@ def handle_event(event: l0.ObservabilityEvent):
         case l0.ObservabilityEventType.RETRY_ATTEMPT:
             print(f"Retrying (attempt {event.meta.get('attempt', '?')})")
         case l0.ObservabilityEventType.FALLBACK_START:
-            print(f"Switching to fallback {event.meta.get('index', '?')}")
+            print(f"Switching to fallback {event.meta.get('toIndex', '?')}")
         case l0.ObservabilityEventType.CHECKPOINT_SAVED:
             print(f"Checkpoint saved ({event.meta.get('token_count', 0)} tokens)")
         case l0.ObservabilityEventType.NETWORK_ERROR:

DETERMINISTIC_LIFECYCLE.md

@@ -142,7 +142,7 @@ This document specifies the **deterministic lifecycle behavior** of the L0 Pytho
 2. [error occurs, retries exhausted]
 3. RETRY_GIVE_UP
 4. on_error(error, will_retry=False, will_fallback=True)
-5. FALLBACK_START (index=1, fromIndex=0, reason="previous_failed")
+5. FALLBACK_START (fromIndex=0, toIndex=1, reason="previous_failed")
 6. FALLBACK_MODEL_SELECTED (index=1)
 7. on_fallback(0, "previous_failed")  # 0-based fallback index
 8. on_start(1, False, True)
@@ -359,7 +359,7 @@ The following `ObservabilityEventType` values are emitted during the lifecycle:
 
 | Event Type                | Description                              | Meta Fields            |
 | ------------------------- | ---------------------------------------- | ---------------------- |
-| `FALLBACK_START`          | Switching to fallback stream             | `index`, `fromIndex`, `reason` |
+| `FALLBACK_START`          | Switching to fallback stream             | `fromIndex`, `toIndex`, `reason` |
 | `FALLBACK_MODEL_SELECTED` | Fallback model selected                  | `index`                |
 | `FALLBACK_END`            | Fallback completed                       | `index`, `success`     |