Before: openTerminal() never closed the previous WebSocket. After A→B→A,
two WS connections to session A both wrote to _term → double keystroke echoes.
Rapid switches triggered a CONNECTING-state send error and reconnect loop.
After: openTerminal() nulls _currentSession, clears reconnect timer, and
closes _ws before opening a new connection. connect() event handlers now
capture the WS instance in a local const and check 'ws !== _ws' on entry
so stale connections silently ignore all events.
Tests: three new regression tests verify:
- openTerminal() calls close() on the previous WS (Bug 1)
- stale open handler does not send on new WS after switch (Bug 2)
- stale close handler does not schedule reconnect after switch (Bug 2)
Brian Krabach
26e2baa89c