Improve accessibility: expose split panes

[barrettj]

Terminal split panes were previously opaque to assistive technology. This PR adds NSAccessibility conformance so that:

• Split layouts expose a split-group container with ordered terminal pane children
• Each pane has a dynamic accessibility label/value (title and working directory) plus a "Pane N of M" hint
• Focus changes post focused-UI-element notifications so VoiceOver and other tools can announce the active pane
• Inactive terminal tabs are hidden from the accessibility tree

Testing:

• Verified in Accessibility Inspector that "Terminal split: N pane(s)" appears as a split group with N AXTerminal children, each with "Pane i of N" help.
• Verified focus and pane count updates.
• Verified switching tabs does not leak other tab panes into the AX tree.

[khoi]

this is great, tysm