Sidebar title-only updates pay full branch/directory/PR snapshot cost on every keystroke

[arzafran]

Tracking upstream cmux manaflow-ai#6546 — applicable to our fork but needs a product/scope decision.

Assessment: Real CPU churn: every updatePanelTitle flush mutates @Published var title (Workspace.swift:7850) → sidebarImmediateObservationPublisher fires (it includes $title at Workspace.swift:6638) → workspaceObservationGeneration &+= 1 in ContentView.swift:13039 → TabItemView.body re-evaluates orderedPanelIds, branchDirectoryLines, and pullRequestRows (ContentView.swift:12684-12720) on every title flush. Split the generation counter into title-only vs detail. Medium refactor touching ContentView.swift and Workspace.swift. No easy unit test.

Upstream: manaflow-ai#6546

[arzafran]

Decision: Build (full refactor).

Verification note: a debounced sidebarObservationPublisher already exists and the expensive snapshots only recompute when aux details are visible — but the immediate publisher still bumps the generation counter on every title flush.

Plan: split workspaceObservationGeneration (ContentView.swift:13039) into title-only vs detail counters; drop $title from sidebarImmediateObservationPublisher (Workspace.swift:6638) into a title-scoped signal. Respect the TabItemView .equatable() typing-latency contract. Cleared needs-decision.

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