#11485 perf(cmux-tui): avoid cloning projection specs per frame

Summary

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

SidebarViewSpec::clone copied the view id, levels, actions, and optional labels on every visible projection render. The scalar extraction removes those per-frame heap copies without changing row ordering, selection, hit targets, or labels. The behavior test guards the rendered semantics. Allocation behavior was reviewed statically because this change has no observable UI difference and local Rust/Cargo execution is prohibited by the worktree instructions.

Verification

• rustfmt --edition 2024 --check cmux-tui/crates/cmux-tui/src/ui/sidebar.rs cmux-tui/crates/cmux-tui/src/app.rs
• git diff --check
• hosted focused test pending

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Summary by cubic

Draws the sidebar projection without cloning the whole view spec each frame, removing per-frame heap copies while keeping row ordering, selection, hit targets, and labels unchanged.

• Extracts only the leaf resource kind before mutable rendering.
• Adds a render test covering the empty projection's leaf label.

Summary by CodeRabbit

• Bug Fixes
  • Corrected the empty sidebar state to display the appropriate “no tabs” label instead of “no workspaces.”
  • Improved consistency when showing empty workspace and tab projections.