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fix(FocusTrap): let only the topmost trap act (#1276)

#1051 replaced the blanket `EventHandler.off(document, EVENT_KEY)` with removal
of each trap's own handlers, so that deactivating one trap stops unhooking the
others. That part was right and stays. What went with it was the property the
blanket had been providing: with only one trap's listeners ever attached, a
focus loop could not form.

Without it, two traps over disjoint elements each pull focus back into their own
`trapElement` whenever it lands outside, so they hand it to each other with no
end condition. Measured on a probe capped so it could terminate: 68 handler
calls and climbing, against 2 for the shape that still had the blanket.

The reach here is wider than the one component: `util/popup.ts` traps focus for
DatePicker, DateRangePicker, DateTimePicker, TimePicker and everything on
`combobox-base`, so two disjoint traps are one open picker plus one open
combobox.

An array of active traps restores the guarantee without giving up nesting: every
trap keeps its own listeners, and both handlers return early unless the instance
is the last one activated. Focus settles in the newest trap, as it did before
#1051, and the one underneath takes over again when the top deactivates — which
the blanket could never do.

Tests: focus does not ping-pong between two disjoint active traps (red without
the check), and control returns to the trap underneath when the top one is
deactivated.

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