fix(cmux-next): restore workspace title motion tunable
Merged
TE
18d ago+1-5
Restores the shared SidebarWorkspaceTitleMotionVariant and workspaceTitleMotion tunable removed by #21407 while WorkspaceRowView and SidebarTunablesWatch still reference them. Keeps the existing crossfade default and fixed title-slot animation contract.
Validation: diff --check; exact one-file repair. Native compilation remains fleet CI-only.
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Summary by cubic
Restores the SidebarWorkspaceTitleMotionVariant and workspaceTitleMotion tunable removed by #21407 while WorkspaceRowView and SidebarTunablesWatch still reference them. The enum cases and defaults are unchanged; only the explanatory comments are trimmed.