fix(plugins): copy history entries out, lock clear_state

Review follow-ups on the transition history.

get_state_history() copied only the outer list, so a caller holding a
returned transition could rewrite the manager's record of what happened
-- which contradicted the defensive-copy guarantee in its own docstring.
Copy each entry too. Every value in a transition is immutable, so a
shallow copy per entry is enough. test_get_state_history_entries_are_copies
pins it; without the change it fails with 'tampered' == 'enabled'.

clear_state() mutated five shared dicts without holding _lock, while
every other mutator takes it. A concurrent set_state() could interleave
and leave a plugin with history but no state. Drop the five as one unit.

This does not close the wider unload-vs-worker race, which lives in
PluginManager.unload_plugin() and predates this change: an update worker
still in flight can call set_state() after clear_state() returns and
recreate the entry. Serialising that needs the per-plugin lock held
across worker join in unload_plugin(), which is a separate change.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

test/test_plugin_state_history_cap.py

@@ -132,6 +132,24 @@ def test_get_state_history_returns_a_copy():
     assert len(manager.get_state_history("clock")) == 1
 
 
+def test_get_state_history_entries_are_copies():
+    """Copying the outer list is not enough -- the entries are handed out too.
+
+    A caller holding a returned transition must not be able to rewrite the
+    manager's record of what happened.
+    """
+    manager = PluginStateManager()
+    manager.set_state("clock", PluginState.ENABLED)
+
+    entry = manager.get_state_history("clock")[0]
+    entry["to"] = "tampered"
+    entry["error"] = "injected"
+
+    stored = manager.get_state_history("clock")[0]
+    assert stored["to"] == PluginState.ENABLED.value
+    assert stored["error"] is None
+
+
 def test_clear_state_drops_history():
     """Unloading a plugin still releases everything it accumulated."""
     manager = PluginStateManager()