Improve tab lifecycle performance

[theanotherwise]

Tuning for:

• improved loading tabs from a file as iTerm2 tabs
• optimized closing behavior when many tabs are open
• improved tab switching/navigation with large numbers of open tabs
• improved rapid tab creation when holding the new-tab shortcut
• reduced redundant window-wide updates during bulk tab operations

[theanotherwise]

@gnachman

[gnachman]

Thanks for this, the performance wins here are real and I want them. My main concern is scope: the PR bundles several independent efforts (vertical tab virtualization, the bulk-operation deferral in PseudoTerminal, lazy progress indicators, title-update debouncing) together with some small, obviously-safe refactors. That makes the risky parts hard to review with confidence because they're interleaved with the trivial ones.

To shrink the diff, I've pulled three of the safe, self-contained changes straight to master:

• 393f7dc42 - fork child processes off the main thread (the legacy/monoserver dispatch_sync -> dispatch_async; also fixes the monoserver completion passing an unassigned savedStatus/nil instead of the real status + error code)
• e917e2a92 - hoist the repeated [aTab sessions] calls in the tab close-confirm path
• 1a0e706da - early-return in -[PTYTab setObjectCount:] when the count is unchanged

If you rebase on latest master these should drop out of your diff.

Could you split the rest into separate PRs? Roughly: (1) the vertical-tab virtualization in PSMTabBarControl/styles/drag assistant, and (2) the bulk-operation deferral in PseudoTerminal/PTYTab/PTYTabView. Those are each big and independent enough to want their own before/after profiling and their own review. The lazy-indicator and title-debounce changes are also separable if you'd rather land them incrementally.

One heads-up: -[PTYTab isProcessing] and -setLabelAttributesForActiveTab: drop the ![self isForegroundTab] guard, which is a user-visible behavior change (activity spinner now animates on the foreground tab), not a cleanup. Let's discuss that one on its own rather than fold it in.

[theanotherwise]

@gnachman i will prepare soon