Retain dynamic logger caller discovery

Keep the `sys.modules` lookup as the normal fast path, then use
`inspect.getmodule()` for unregistered `runpy`, plugin, and `exec()`
namespaces.

Cover an unregistered module name backed by a real package file and
verify implicit logger naming still resolves to that package.

Review: PR #478 (goodboy)
https://github.com/goodboy/tractor/pull/478#pullrequestreview-4922213201

(this patch was generated in some part by `opencode` using
`gpt-5.6-sol` (`openai`))

tests/test_log_sys.py

@@ -2,9 +2,11 @@
 `tractor.log`-wrapping unit tests.
 
 '''
+import importlib
 import logging
 from pathlib import Path
 import shutil
+import sys
 from types import ModuleType
 
 import pytest
@@ -165,6 +167,53 @@ def test_implicit_mod_name_applied_for_child(
     assert submod.log.logger in sub_logs
 
 
+def test_implicit_mod_name_from_unregistered_namespace(
+    tmp_path: Path,
+):
+    '''
+    Preserve implicit logger naming for dynamic module namespaces.
+
+    The fast `sys.modules` caller lookup cannot resolve `runpy`,
+    plugin-loader, or `exec()` namespaces that are not registered.
+    Compile a real package file under an unregistered module name so
+    the rare filename fallback must recover its imported package and
+    retain the same package-level logger name.
+
+    '''
+    pkg_name = 'dynamic_logger_pkg'
+    pkg_dir = tmp_path / pkg_name
+    pkg_dir.mkdir()
+    init_path = pkg_dir / '__init__.py'
+    init_path.write_text('')
+    mod_path = pkg_dir / 'plugin.py'
+    mod_path.write_text('')
+
+    sys.path.insert(0, str(tmp_path))
+    try:
+        importlib.import_module(pkg_name)
+        namespace = {
+            '__name__': f'{pkg_name}.unregistered',
+            '__package__': pkg_name,
+            'tractor': tractor,
+        }
+        exec(
+            compile(
+                'log = tractor.log.get_logger('
+                f'pkg_name={pkg_name!r})',
+                str(mod_path),
+                'exec',
+            ),
+            namespace,
+        )
+        dynamic_log = namespace.get('log')
+    finally:
+        sys.path.remove(str(tmp_path))
+        sys.modules.pop(pkg_name, None)
+
+    assert dynamic_log is not None
+    assert dynamic_log.name == pkg_name
+
+
 def test_io_custom_level_registered():
     '''
     The `IO`(21) level (registered via `add_log_level()` at

tractor/log.py

@@ -463,7 +463,14 @@ def get_logger(
         if mod_name is None:
             return None
 
-        return sys.modules.get(mod_name)
+        if caller_mod := sys.modules.get(mod_name):
+            return caller_mod
+
+        # Preserve caller discovery for `runpy`, plugin loaders,
+        # and `exec()` namespaces not registered in `sys.modules`.
+        # Import `inspect` only on this rare fallback path.
+        from inspect import getmodule
+        return getmodule(caller_frame)
 
     # --- Auto--naming-CASE ---
     # -------------------------