macOS 26.5 SDK adds NSViewController.isSelectable, breaking subclasses

Building with Xcode 26.6 / macOS 26.5 SDK, an NSViewController subclass declaring private let isSelectable: Bool fails with 'cannot override with a stored property isSelectable', 'overriding property must be as accessible as its enclosing type', and 'ambiguous use of isSelectable'. The same code compiles against the macOS 26.0 SDK (Command Line Tools), so local swiftc typechecks can't catch it. Rename the stored property (e.g. allowsTextSelection). In general, avoid generic AppKit-sounding names for stored properties on NSView/NSViewController subclasses, and typecheck with the same SDK version as CI.