The armed tool per pointer-mode ('pointer'/null = the mode default), keyed by
mode id. The rail slot reflects this: a non-default modal tool morphs the slot's
icon + label. Reset to the mode default when its mode is left.
The active rail mode. Defaults to 'select'.
Whether each pointer-mode's contextual tool panel is expanded, keyed by mode id. Remembered per mode (persists across mode switches); the armed tool does not.
Observable state of the mode rail.