One shared field contract on a base class, not a mixin or controller

Every value-bearing control extends JollyField, which owns label, value, default, lockedBy, peers, path, disabled, readonly, error, align and labelPosition — plus the chrome rendered around the value. The four state-bearing members are in from the start because they change what a value means, and retrofitting them touches every render path.

Mixed marks a value that differs across a multi-selection. It is declared as Symbol.for("jolly-pixel.ui.mixed") as unique symbol: the registry lookup preserves identity across duplicate module instances, and the cast preserves the distinct type for narrowing.

Considered Options

• A reactive controller. A controller cannot render the chrome around the value, so wiring lockedBy would edit twelve files instead of one.
• A generic mixin. Loses T, which then has to be redeclared per control.
• jolly-property-row owning label, gutter and chips. Every field would need a wrapper.
• Symbol("mixed") for the sentinel. Identity breaks silently across a duplicate module instance, and ui is resolved through dist/ by three editors.

Consequences

JollyField stays out of the public barrel. Exporting it publishes a subclassable base, its protected surface and its DOM shape as versioned API before any consumer subclasses it; promoting it later is additive.