#11487 perf(cmux-tui): avoid cloning projection specs per frame

Summary

• extract the SidebarResourceKind leaf before mutable projection rendering
• preserve empty-projection and missing-view behavior
• add a behavior test for the configured leaf label

This replaces https://github.com/manaflow-ai/cmux/pull/11485 under the matching opener and commit-author identity. The source head is unchanged. The scalar extraction removes per-frame SidebarViewSpec heap copies without changing row order, selection, hit targets, or labels.

Verification

• rustfmt --edition 2024 --check
• git diff --check
• exact hosted Linux, macOS, MSRV, and aarch64 proof: https://github.com/manaflow-ai/cmux/actions/runs/33546180678
• exact stable Codex 0.151, gpt-5.6-sol, high review: clean, correctness 0.98

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Summary by cubic

Avoids cloning the projection spec on every frame, cutting per-frame heap allocations in the sidebar without changing row order, selection, hit targets, or labels.

• Derives the empty-projection resource label from a borrowed view spec instead of a cloned one.
• Adds a regression test that ensures an empty projection uses its configured leaf resource label.

Summary by CodeRabbit

• Bug Fixes

  • Empty sidebar views now display the correct message for the configured resource, such as “no tabs,” instead of incorrectly referring to workspaces.

• Tests

  • Added coverage to verify empty-state messaging when a sidebar view is configured with multiple levels.