[Victor Genaev (mainframev)]

Stack: #36811 scaffold (merged) → #36845 spec → #36812 time utilities → #36813 TimePicker → #36814 headless TimePicker

Previous Behavior

The compat TimePicker:

• wraps the styled useCombobox_unstable and mutates its state;
• uses a non-standard onTimeChange(event, data) callback;
• types formatDateToTimeString as (date) => string, although it is called with format options.

New Behavior

Adds a v9 TimePicker built on Combobox with the same behavior:

• Selection follows the v9 form control convention: value / defaultValue (Date | null), optional displayValue for controlled input text, and onChange: EventHandler<TimePickerOnChangeData> firing on commit. The data is a discriminated EventData union (click | keydown | change | blur), so type narrows event. The native input value, defaultValue and onChange are omitted from the props.
• The input text is derived from value with formatDateToTimeString, so it no longer has to be controlled alongside the selection. Typed freeform text is kept after a commit so the user can correct it.
• The default dateAnchor is captured once from the initial value and does not move when a controlled selection is accepted.
• Leaving the input after clearing no longer reports required-input.
• useCustomStyleHook_unstable('useTimePickerStyles_unstable') is applied; the hook name is registered in react-shared-contexts (minor).
• formatDateToTimeString(date, options) gets the options it is called with.
• useTimePickerBase_unstable returns state with no appearance/size, default icons or Tabster.
• The time logic is split into two hooks that work with any Combobox state hook: useTimePickerSelection_unstable (options, selection, Combobox props) and useTimePickerComboboxState_unstable (freeform commit on Enter or blur, clear handling, Field labelling). The headless package reuses them in feat(react-headless-components-preview): add headless TimePicker #36814 without pulling in react-positioning.
• Combobox state is no longer mutated after it is created; callbacks and refs are merged instead.
• The Field-labelled chevron now also gets the id that its aria-labelledby points to.
• Time utilities are exported.

Other props (startHour, endHour, increment, dateAnchor, hourCycle, showSeconds, freeform, parseTimeStringToDate) are unchanged. Compat's selectedTime / defaultSelectedTime / onTimeChange map to value / defaultValue / onChange, see the spec's migration table.

The compat unit and Cypress tests are ported, and base-hook tests are added. Stories are ported under Preview Components/TimePicker, and the package is added to public-docsite-v9

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