tracking: mouse interaction on the tab bar

[GeneralD]

Overview

Tracking issue for mouse interaction on the tab bar. The bar started as a passive picture; over several features it is becoming a clickable surface. They all build on the same foundation, so grouping them keeps the shared design (and its traps) in one place.

Shared foundation

Every mouse feature reuses the same two pieces:

• Hit-test geometry — LineLayout { tabs, left, right } (src/line.rs:47-51) and TabHit { position, start, width, active } (src/line.rs:24-31) record what each render drew; position_at_column / switch_target_at_column resolve a click column to a target (src/line.rs:111, src/lib.rs:301-310). New click targets extend this recorded geometry rather than overprinting.
• Permission model — the default request is exactly ReadApplicationState + ChangeApplicationState (src/lib.rs:291-298). Grants are all-or-nothing and cached, so adding a permission freezes the bar for existing users until they re-grant (zellij#4982). Any new action that needs a third permission must be gated behind an opt-in config flag, the way reorder gates RunActionsAsUser (feat(config): gate RunActionsAsUser behind a reorder flag (default off) so auto-updaters don't freeze #23).

Cross-cutting concerns the children share: row-aware hit-testing (the bar is multi-row, so a click target on one row must not steal clicks from another), width budgeting (new affordances must be reserved in LineLayout, not overprinted), and graceful no-op (clicks on padding / overflow markers / degraded blocks must do nothing rather than misfire).

Done (foundation)

• mouse: click a tab block to switch tabs #8 — click a tab block to switch tabs (column hit-test, the original tab_layout consumer)
• v2: drag-and-drop tab reordering #10 — drag-and-drop tab reordering (grab / hold / release state machine, v2)
• mouse: click a minimap pane to focus it (position -> pane hit-test) #74 — click a minimap pane to focus it (position -> pane hit-test; per-pane sub-rectangles)
• mouse: scroll the tab bar to switch tabs or walk panes (tab/pane modes) #80 — scroll the wheel to switch tabs (tab) or walk panes across tabs (pane)

To do

• mouse: a '+' button at the end of the tab strip to open a new tab #76 — a "+" button at the far right to open a new tab (verify new_tab() permission first)
• scroll: configurable wheel sensitivity (too fast — one flick jumps several steps) #83 — configurable wheel sensitivity (scroll feels too fast — refines mouse: scroll the tab bar to switch tabs or walk panes (tab/pane modes) #80)
• mouse: an 'x' close button on each tab block (click hit-test) #86 — an "x" button on each tab block to close it (close_tab_with_index)

Notes

• This is a meta/tracking issue; the actual work and acceptance criteria live in each child.
• New mouse ideas (right-click context menu, middle-click close, scroll-to-switch, etc.) can be filed as children and added to the To do list above.