#22898 cmux-next sidebar: public import Observation in ChiefSidebar (base compile red since #22759)

Base red, priority zero. ChiefSidebar is a public @Observable class. Under InternalImportsByDefault the macro's public Observable conformance needs public import Observation. Every CmuxNext package build of feat-cmux-next fails with cannot use protocol 'Observable' in a public or '@usableFromInline' conformance; 'Observation' was not imported publicly (cmux-ci step dcf9d85d09a1fe772bf22fdf, aws-m4pro-3). No open PR fixes it (#22852 touches the file's imports but keeps import Observation).

Changelog

none

Observed / Not run

• Observed: the compile error in the step log at the base.
• Not run: a separate compile of this one-word change.

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Summary by cubic

Fixes the compile error on the feat-cmux-next base by making the Observation import public in ChiefSidebar.swift.

Since ChiefSidebar is a public @Observable class, the macro's public Observable conformance requires public import Observation under InternalImportsByDefault. Without this change, every CmuxNext package build fails with "cannot use protocol 'Observable' in a public or '@usableFromInline' conformance; 'Observation' was not imported publicly."