Right pane: Ctrl+1…9 jump to tabs

[and-human]

Summary

Number shortcuts for the right pane's tabs, the way cmux pairs Cmd+1…9 (workspaces) with Ctrl+1…9 (the tabs inside a pane). Zeron already has the first half: ⌘1…9 jumps sidebar sessions. With several side chats, subagent transcripts, files and a browser open next to a chat, the only way between them today is the mouse, or Ctrl+Tab stepping one at a time once focus is already in the pane.

• ⌃1…⌃8 select right pane tab 1…8 of the current chat, in tab-strip order (drag-reorders respected). ⌃9 selects the last tab, as in browsers.
• If the pane is closed it opens on that tab. A number past the last tab does nothing. Like Ctrl+Tab, the jump is quiet under keyboard-owning overlays and only applies on the chat route.
• Focus follows activation exactly as Ctrl+Tab's does (activate_right_surface): a side chat lands in its composer, a file in its editor, a browser in its address bar.
• Off macOS the default is Alt+1…9. There mod is Ctrl, so Ctrl+1…9 already belongs to the session jumps. This is the same platform split the session-cycling shortcuts use.
• Rebindable like every other shortcut: ShortcutId::JumpRightTab(slot), persisted as keymap.jumpRightTab: [...], which heals by slot like jumpSession. It gets its own "Jump to right pane tab" group on the Shortcuts page.

Please look closely at:

• Defaults on macOS. ⌃1…9 is also macOS's "Switch to Desktop N", which is off by default. A user who has it enabled can rebind on the Shortcuts page, which already flags conflicts.
• Terminal tabs. A matched app binding wins over a focused terminal, so ⌃1…9 switches tabs rather than reaching the shell. Terminals don't send anything distinct for Ctrl+digit, so I kept the panel behavior. Happy to exclude terminal focus instead if you prefer.
• Compatibility. Settings-only (ui-settings.json, serde-defaulted). Older files load with the new defaults and keep their own combos. No wire or doc types change.
• Not included: number badges on the tabs while the modifier is held, like the sidebar's jump hints. That touches the tab strip's rendering, so it would be a separate PR.

Test plan

• settings::tests::right_tab_jumps_default_per_platform_without_conflicts: macOS ⌃N, elsewhere Alt+N; every default unique on both platforms; defaults spelled as each platform's recorder stores them; set, reset, and out-of-range slot
• settings::tests::right_tab_slots_pick_their_tab_and_the_last_slot_the_last_tab: slot to tab index, last slot to last tab, empty and past-the-end
• settings::tests::older_keymaps_gain_the_right_tab_jumps: a pre-existing keymap keeps its combos and gains the defaults; a short list heals by slot
• shell::…::right_tab_jumps_select_by_strip_order: real keystrokes through apply_keymap select tabs 2, 1, then the last; past the end is a no-op; a closed pane reopens on the chosen tab
• Existing: every_shortcut_lands_in_a_rendered_group, every_shortcut_default_is_unique_and_bindable, conflict_detection, the session-jump tests, and navigation_shortcuts_work_from_settings
• cargo test --locked -p zeron-ui --lib -- --test-threads=1
• cargo clippy -p zeron-ui --all-targets: nothing new in touched code. rustfmt applied to my changes only.

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