fix: expand Ink partial clears to prevent ghost content in screenshots

Ink (React CLI framework) clears its output using repeated EL2+CUU1
sequences, one per previously-drawn line. When /clear resets Ink's
internal line counter, the next frame only erases a few lines instead
of the full previous output. In a real terminal the old content is in
scrollback and invisible, but pyte's fixed-size screen retains it,
producing ghost content (e.g. duplicated prompts) in SVG screenshots.

Added AltScreen.expand_clear_sequences() which detects runs of 3+
EL2+CUU1 pairs that don't reach row 0 and extends them to erase all
lines up to the top of the screen. Both DockerExecSession and
TerminalSession call this before feeding data to pyte.

Also made on_session_end() idempotent (contextlib.suppress KeyError)
to prevent a race when close_session() and natural session exit both
call it.

Added docs/ink-clear-fix.md with root cause analysis, byte-level
explanation, and reproduction script.

tests/test_session_manager.py

@@ -105,6 +105,21 @@ class TestSessionManager:
         assert session_id not in manager.sessions
         assert route_key not in manager.routes
 
+    def test_on_session_end_idempotent(self, mock_poller, mock_path, sample_apps):
+        """Test session end cleanup is idempotent."""
+        manager = SessionManager(mock_poller, mock_path, sample_apps)
+
+        session_id = SessionID("test-session")
+        route_key = RouteKey("test-route")
+        manager.sessions[session_id] = MagicMock()
+        manager.routes[route_key] = session_id
+
+        manager.on_session_end(session_id)
+        manager.on_session_end(session_id)
+
+        assert session_id not in manager.sessions
+        assert route_key not in manager.routes
+
     def test_on_session_end_nonexistent(self, mock_poller, mock_path, sample_apps):
         """Test session end for non-existent session."""
         manager = SessionManager(mock_poller, mock_path, sample_apps)