Fix stepper step callbacks, PivotViewer grouping and transitions, and non-modal ChatSidebar rendering

[woksin]

Fixed

• CommandStepper and StepperCommandDialog now call onChangeStep({ index }) once for every accepted move to a different step: header clicks, Previous, and Next. Previous and Next never reported a change before, and a header click reported one even when the move was then rejected. (CommandStepper/StepperCommandDialog lack ARIA tab semantics and a step-changed callback for Next/Previous #249)
• Each stepper panel is now labelled by its step header through aria-labelledby. If you pass pt.header.id, your id is kept and the panel keeps its text label. The wizard stays an ordered list of steps rather than ARIA tabs. (CommandStepper/StepperCommandDialog lack ARIA tab semantics and a step-changed callback for Next/Previous #249)
• PivotViewer boolean dimensions now group into true and false columns, and their categorical filters narrow the cards, including the None and Unknown options for missing values. (Honor PivotViewer dimension grouping contracts and localize toolbar copy #334)
• PivotViewer keeps missing values missing instead of turning them into the text undefined, and infers each column's type from its first non-missing value. Missing numbers sort last. Because of this, a numeric column whose first value is missing is now treated as numeric, so a categorical selection on it no longer narrows the cards, as for every other numeric column. (Honor PivotViewer dimension grouping contracts and localize toolbar copy #334)
• PivotViewer now applies a dimension's declared formatValue to column labels and sort to column order. Changing either no longer re-runs the grouping. (Honor PivotViewer dimension grouping contracts and localize toolbar copy #334)
• PivotViewer no longer renders an empty viewport after regrouping or filtering when the new layout's first entries are off-screen. Cards that intersect the viewport are drawn during the transition, and cards that move off-screen are released when it ends. Refs Clicking around in pivot viewer sometimes crashes it #236: the intermittent crash reported there is not reproduced, so that issue stays open.
• A non-modal ChatSidebar can now be rendered open on the server. It portals after mount into the CratisComponentsProvider overlay container, and falls back to document.body when there is none. (Non-modal ChatSidebar breaks server rendering and skips its open/close animation #339)
• The non-modal ChatSidebar now plays the same open and close animation as the modal one, and skips it when the user prefers reduced motion. (Non-modal ChatSidebar breaks server rendering and skips its open/close animation #339)

Changed

• yarn ci in Source now also runs the public API TSDoc check and the packed-archive CSS budget check, which previously ran only in GitHub CI.