fix(plugins): put a (re)loading plugin's directory first on sys.path (#663)

Plugins import their own files by bare name (`from sports import ...`),
which resolves to the first directory on sys.path that has the file. The
loader added a plugin's directory only if it was missing, so on a reload --
a live re-enable from the web UI -- the plugin's directory stayed behind
every plugin loaded since, and its bare imports found their files first.

Seen on ledpi: re-enabling UFC with hockey running failed with "cannot
import name '_status_is_final' from 'sports'" (it got hockey's sports.py).
A loading plugin's directory is now always moved to the front. Every
scoreboard ships its own sports.py, so any of them was exposed on reload.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

test/test_plugin_loader_reload_isolation.py

@@ -0,0 +1,60 @@
+"""A reloaded plugin imports its own bare-name modules, not another plugin's.
+
+Scoreboard plugins each ship a ``sports.py`` and import it by bare name. Each
+plugin's directory goes on sys.path when it loads, and a bare import resolves
+to the first directory that has the file. The loader used to add a directory
+only if it was missing, so after alpha, then beta, loaded, re-enabling alpha
+from the web UI left beta's directory in front: alpha's ``from sports import
+...`` got beta's copy. On a Pi, re-enabling UFC with hockey running failed
+with "cannot import name '_status_is_final' from 'sports'".
+"""
+
+import sys
+
+import pytest
+
+from src.plugin_system.plugin_loader import PluginLoader
+
+
+def _write_plugin(root, name):
+    d = root / name
+    d.mkdir(parents=True)
+    (d / "sports.py").write_text(f"WHO = {name!r}\n", encoding="utf-8")
+    (d / "manager.py").write_text("from sports import WHO\n", encoding="utf-8")
+    return d
+
+
+@pytest.fixture
+def plugins(tmp_path):
+    before_path = list(sys.path)
+    before_modules = set(sys.modules)
+    dirs = {name: _write_plugin(tmp_path, name) for name in ("alpha", "beta")}
+    yield dirs
+    sys.path[:] = before_path
+    for key in set(sys.modules) - before_modules:
+        sys.modules.pop(key, None)
+
+
+def _unload(loader, plugin_id):
+    # What PluginManager.unload_plugin does to the module entries.
+    sys.modules.pop(f"plugin_{plugin_id}", None)
+    loader.unregister_plugin_modules(plugin_id)
+
+
+def test_each_plugin_gets_its_own_bare_module(plugins):
+    loader = PluginLoader()
+    assert loader.load_module("alpha", plugins["alpha"], "manager.py").WHO == "alpha"
+    assert loader.load_module("beta", plugins["beta"], "manager.py").WHO == "beta"
+
+
+def test_a_reloaded_plugin_still_gets_its_own_bare_module(plugins):
+    loader = PluginLoader()
+    loader.load_module("alpha", plugins["alpha"], "manager.py")
+    loader.load_module("beta", plugins["beta"], "manager.py")
+
+    _unload(loader, "alpha")
+    reloaded = loader.load_module("alpha", plugins["alpha"], "manager.py")
+
+    assert reloaded.WHO == "alpha"
+    assert sys.path.index(str(plugins["alpha"])) < sys.path.index(str(plugins["beta"]))
+    assert sys.path.count(str(plugins["alpha"])) == 1