fix: readable, wheel-scrollable tabs and no tab bar for a single tab

[arzafran]

What this does

People who keep dozens of tabs open in one pane could barely read them: every tab shrank to the same width and showed one letter, and only a sideways trackpad swipe scrolled the strip. Tabs now keep a width that fits their title, a regular mouse wheel scrolls the strip, and the selected tab stays in view. A pane with a single tab hides its tab bar entirely, the way cmux does, and the bar comes back at two tabs.

Summary

Review order: Sources/WindowPaneChromePortal.swift, then PaneContainerView.swift.

• Clipped titles. layoutPills() squeezed every pill to an even share once the row overflowed, floored at 78 pt, which left about 10 pt for the title. The compression is gone: pills take min(220, max(78, title width)) and the strip scrolls.
• Wheel scrolling. A private TabStripScrollView turns a phase-less, vertical-only wheel event into horizontal scroll. Trackpads and Magic Mouse keep native handling. No global event monitors.
• Selected tab in view. On a selection change the native bar centers the selected pill once, so manual scrolling is not undone by later layout passes.
• Stale titles. TabItem == compares ids only, so SwiftUI skipped updateNSView on a rename or OSC title change. BonsplitPaneChromeAnchor now gets a content signature of the tab fields the bar draws.
• Single-tab panes. showsTabStrip is false when a pane has exactly one tab, except for the top-left pane in minimal mode with the sidebar hidden, where the strip is the titlebar row next to the traffic lights and keeps the window draggable.
• Controls. When the focused pane's strip is hidden, the new-tab and split capsules hide instead of jumping to another pane, where they would act on the wrong one.
• Removed the unused Appearance.tabMinWidth and tabMaxWidth from BonsplitConfiguration (no readers).

Test Plan

• ./scripts/reload.sh --tag tabs-overflow builds.
• Release arm64 xcodebuild ends in BUILD SUCCEEDED.
• python3 scripts/check-structural-budgets.py passes.
• Tagged app, 1 tab: bar hidden. 2 tabs: bar shown. 30 tabs with mixed titles: titles readable.
• Synthesized line-unit wheel events scroll the strip sideways.
• rename-tab updates the pill (one paneChrome.update log line per rename; none before the fix).
• Real mouse wheel and trackpad check by hand.
• Standard titlebar mode with a single tab: window still drags from the titlebar band (reasoned from code, not exercised).
• CI green.

Screenshots: before/after needed (30 tabs, single tab).