Functional test coverage gaps: viz packages (charts/vega/lab) at ~0%, plus untested core hooks

## Problem

A sweep of the shippable packages (`core`, `lab`, `charts`, `vega`) found that **functional test coverage is uneven**: `core` is well-covered, but the visualization packages are almost entirely untested, and a cluster of core hooks have no direct tests.

"Functional" here = a test that renders + asserts, drives interaction (`fireEvent`/`userEvent`), queries the DOM (`getByRole` etc.), or exercises a hook via `renderHook` — not type-only, snapshot-only, or codemod string-match tests.

## Findings (public exports)

| Package | Components func-tested | Hooks func-tested |
|---|---|---|
| `core` | 161 / 173 (**93%**) | 44 / 64 (**69%**) |
| `lab` | 16 / 52 (**31%**) | 1 / 2 |
| `charts` | 0 / 6 (**0%**) | 0 / 1 |
| `vega` | 0 / 1 (**0%**) | — |

**Total gap: ~55 components + ~22 hooks** with no functional test.

### 1. Visualization packages are the biggest hole (`lab` charts, `charts`, `vega`)
The only viz tests today cover pure-math helpers (color scales, projection, range) plus one `SankeyChart` render. Every actual chart-primitive component renders nothing under test — all `Chart*`, `Radial*`, `ThreeD*`, `Sankey{Grid,Label,Link}`, `SVGIcon` in `lab`; the whole `charts` primitive set (`ChartAxis/Grid/Legend/Swatch/Tooltip`); and `VegaChart`.

### 2. Core hooks lack direct tests (logic-heavy, high value)
20 exported core hooks have no dedicated test. Many run indirectly when their parent component renders (e.g. `useTooltip` via `Tooltip.test.tsx`), but none isolate the hook's own logic/edge cases via `renderHook`. Examples: `useTooltip`, `useHoverCard`, `useLightbox`, `useCollapsible`, `useScrollLock`, `useScrollOverflow`, `useTruncation`, `useTranslator`, the three `useCalendar*` hooks, `useTableFilterState`.

### 3. A handful of core components have zero tests
`ChatComposerDrawer`, `ChatSendButton`, `CommandPaletteEmpty`, `InputClearButton`, `MobileNavToggle`, `PowerSearchFilterEditor`, `PowerSearchToken`, `TableHeaderCell`, `TypeaheadItem`.

## Relationship to #4163
Complementary, not overlapping. #4163 (contract testing & drift prevention) guarantees the *written contract* (props/types/theming) stays in sync with code, and notes core already has good raw coverage. This issue is about the **raw functional-coverage gap** in the areas #4163 doesn't reach — primarily the viz packages (0%) and untested hooks.

## Proposed approach
1. **Core hooks first** — fast wins: `renderHook` tests isolating each hook's logic and edge cases.
2. **Viz smoke harness** — a shared render-smoke suite for chart primitives (biggest count, currently 0%), then grow into behavior tests.
3. **Zero-test core components** — colocated `.test.tsx` following the existing create-component convention.

Full machine-readable inventory (every component/hook with a func-tested flag) is available and can be attached.