Document {bind}-object emptyValue store seeding with spec and litmus repro

[nebojsa-peric]

Form fields initialize their store slot differently depending on how the value binding is authored:

• A binding object (value={{ bind: "..." }}, including the value-bind shorthand) seeds the path with the field's declared emptyValue (null) the moment the widget initializes — StructuredSelector.getSelectorConfig collects the declared prop default into defaultValues and init() writes it to the store.
• An accessor-chain binding (value={model.x}) takes the isAccessorChain branch, which registers no default, so the slot stays undefined until the user interacts.

An untouched field therefore leaves null or undefined in the store based solely on binding style. Any code that strict-compares such a slot — e.g. disabled={expr(m.a, m.b, (a, b) => a === b)} with two empty slots — flips its result after a seemingly cosmetic refactor from {bind: ...} to an accessor chain, which is how we ran into this.

This PR adds a spec (Field value binding initialization, 4 cases — including one showing the seeded value follows a custom emptyValue, pinning down the mechanism) and a litmus page (bugs/FieldBindingObjectNullSeeding) with a live store dump. No behavior changes — the spec documents the current semantics; whether the asymmetry is intended is up for discussion.

[nebojsa-peric]

Tracking the fix in #1337.