Use `sys._getframe()` in `get_logger()` mod lookup

`get_caller_mod()` (nested in `get_logger()`) walks the WHOLE
call-stack via `inspect.stack()`, which also resolves src-file
info for every frame and scans all of `sys.modules` per frame
via `inspect.getmodule()`. During nested imports (deep
importlib stacks) each module-level `get_logger()` call costs
~5-10ms, making the ~39 such calls dominate `import tractor`
wall-time: ~244ms of the ~420ms total (see gh #470).

Deats,
- resolve the caller frame with `sys._getframe(frames_up)` and
  map its `f_globals['__name__']` through `sys.modules`: O(1)
  vs. O(stack x sys.modules).
- guard `ValueError` (stack too shallow) -> `None`, matching
  the existing null-caller handling at all use-sites.
- drop the now-unused `inspect` imports; pull `FrameType` from
  `types` instead.

Results: `import tractor` drops 0.42s -> ~0.155s; sequential
`.start_actor()` spawn latency ~0.42 -> ~0.18s/actor.

Prompt-IO: ai/prompt-io/claude/20260702T155626Z_65bf9df5_prompt_io.md
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

tractor/log.py

@@ -26,11 +26,6 @@ built on `tractor`.
 '''
 from collections.abc import Mapping
 from functools import partial
-from inspect import (
-    FrameInfo,
-    getmodule,
-    stack,
-)
 import sys
 import logging
 from logging import (
@@ -38,7 +33,10 @@ from logging import (
     Logger,
     StreamHandler,
 )
-from types import ModuleType
+from types import (
+    FrameType,
+    ModuleType,
+)
 import warnings
 
 import colorlog  # type: ignore
@@ -438,16 +436,33 @@ def get_logger(
         pkg_name: str =  _root_name
 
     def get_caller_mod(
-        frames_up:int = 2
-    ):
+        frames_up: int = 2,
+    ) -> ModuleType|None:
         '''
-        Attempt to get the module which called `tractor.get_logger()`.
+        Attempt to get the module which called
+        `tractor.get_logger()`.
+
+        Resolve the caller's frame with `sys._getframe()` and
+        map its `__name__` through `sys.modules`; `inspect.stack()`
+        (the previous impl) builds src-file info for EVERY frame
+        on the stack, scanning all of `sys.modules` per frame via
+        `inspect.getmodule()`, which made module-level
+        `get_logger()` calls dominate `import tractor` time
+        (see gh #470).
 
         '''
-        callstack: list[FrameInfo] = stack()
-        caller_fi: FrameInfo = callstack[frames_up]
-        caller_mod: ModuleType = getmodule(caller_fi.frame)
-        return caller_mod
+        try:
+            caller_frame: FrameType = sys._getframe(frames_up)
+        except ValueError:
+            return None
+
+        mod_name: str|None = caller_frame.f_globals.get(
+            '__name__',
+        )
+        if mod_name is None:
+            return None
+
+        return sys.modules.get(mod_name)
 
     # --- Auto--naming-CASE ---
     # -------------------------