fix(providers): drop loguru, fix FallbackBoundModel failover log placeholders

The `FallbackBoundModel._notify` WARNING used stdlib `%s` positional
substitution but `_log` was loguru when installed, which does not perform
that substitution — so the failover line rendered with literal `%s`
placeholders instead of the resolved provider labels:

    cubepi.providers.fallback: failover triggered  failed=%s  →  next=%s
                                                   reason=%s  attempt=%s/%s

cubepi has never declared loguru as a dependency (see the architecture
notes in `chain_providers` / `_log_tracing_warning`), and hosts that
prefer loguru can intercept stdlib records. Remove the try-import-loguru
branches in `fallback.py` and `base._log_listener_exception`, promote
`logging` to a top-level import in `base.py`, drop the duplicate inline
import in `chain_providers`. The existing `%s`-style log strings now
render correctly through stdlib `logging`.

Author: xfgong

CHANGELOG.md

@@ -86,8 +86,25 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   removed. Existing middlewares that return `None` after one injection are
   unaffected. This enables evaluation loops like `GoalMiddleware`.
 
+### Changed
+
+- **Internal logging now uses stdlib `logging` exclusively.** Previously
+  `FallbackBoundModel` and the provider listener-exception path tried to
+  import `loguru` first and fell back to stdlib. The loguru path was
+  silently incorrect — loguru does not perform `%s` argument substitution,
+  so failover warnings rendered literal `%s` placeholders instead of the
+  resolved labels. cubepi has never declared loguru as a dependency; hosts
+  that prefer loguru should intercept stdlib logging records into it. No
+  public API change.
+
 ### Fixed
 
+- **`FallbackBoundModel` failover log line now substitutes its placeholders.**
+  Before the loguru removal above, the WARNING emitted on every failover
+  read `failed=%s  →  next=%s  reason=%s  attempt=%s/%s` literally because
+  the loguru-backed logger ignored the positional args. Now renders as
+  `failed=anthropic/claude-opus-4-5  →  next=openai/gpt-5  reason=…  attempt=1/2`.
+
 - **`Recorder.attach()` and `Meter.attach()` now subscribe to every provider in
   a `FallbackBoundModel` chain** (closes #167). Previously they only listened
   to `chain[0].provider`, so post-failover calls executed against `chain[1..]`

cubepi/providers/base.py

@@ -3,6 +3,7 @@
 import asyncio
 import copy
 import inspect
+import logging
 from dataclasses import dataclass, field
 from typing import (
     Any,
@@ -555,18 +556,9 @@ async def _safe_run_coroutine(cb: Callable, coro: Any) -> None:
 
 
 def _log_listener_exception(cb: Callable, exc: BaseException) -> None:
-    try:
-        from loguru import logger
-
-        logger.opt(exception=exc).warning(
-            "cubepi provider listener {} raised; swallowed", cb
-        )
-    except ImportError:
-        import logging
-
-        logging.getLogger("cubepi.providers").warning(
-            "cubepi provider listener %r raised; swallowed", cb, exc_info=exc
-        )
+    logging.getLogger("cubepi.providers").warning(
+        "cubepi provider listener %r raised; swallowed", cb, exc_info=exc
+    )
 
 
 def _detach(listeners: list, cb: Callable) -> None:
@@ -967,8 +959,6 @@ def chain_providers(model: object) -> list["BaseProvider"]:
     fallback chain so post-failover provider events land in the trace /
     metric stream.
     """
-    import logging
-
     if model is None:
         return []
     chain = getattr(model, "chain", None)

cubepi/providers/fallback.py

@@ -2,6 +2,7 @@
 
 import asyncio
 import inspect
+import logging
 import time
 from dataclasses import dataclass
 from typing import Any, Awaitable, Callable
@@ -29,12 +30,7 @@
     chain_providers,  # re-exported for back-compat; canonical home is base.py
 )
 
-try:
-    from loguru import logger as _log
-except ImportError:  # pragma: no cover
-    import logging as _logging
-
-    _log = _logging.getLogger("cubepi.providers.fallback")
+_log = logging.getLogger("cubepi.providers.fallback")
 
 
 DEFAULT_TRIGGER_ERRORS: frozenset[type[ProviderError]] = frozenset(