test(selection): assert last_active_visual_selection cleared on disable

Why: DEREM-19 — without this assertion, the disable() clear at
selection.lua:49 could be silently reverted without test failure.

Change-Id: Icefe72a3fb867a541fca10077636673d12f47a04
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Signed-off-by: Thomas Kosiewski <tk@coder.com>

Author: ThomasK33

tests/selection_test.lua

@@ -519,13 +519,15 @@ describe("Selection module", function()
 
       assert(selection.state.demotion_timer == nil)
       assert(selection.state.latest_selection == nil)
+      assert(selection.state.last_active_visual_selection == nil)
       assert.are.equal(1, timer._stop_calls)
       assert.are.equal(1, timer._close_calls)
 
       -- A late-firing callback from the cancelled timer must not mutate state after teardown.
       timer:fire()
       assert(selection.state.latest_selection == nil)
       assert(selection.state.demotion_timer == nil)
+      assert(selection.state.last_active_visual_selection == nil)
       assert.are.equal(1, timer._stop_calls)
       assert.are.equal(1, timer._close_calls)