[r0b1n]

Stacked on #2499 (feat/combobox-selector-safety-net). Research branch for the headless combobox; see docs/design/headless-widgets.md and docs/design/headless-combobox.md, both added here. #2499 stays a plain test PR and carries nothing from this one.

Migration steps 1 and 2 of the combobox design doc, as commits:

0. docs: add headless widgets design docs — the two design documents.
1. build(combobox-web): add mobx, mobx-kit and brandi, run jest in strict action mode — dependencies, jest.config.js + jest.setup.ts with enforceActions: "always" and [MobX] warnings turned into errors.
2. refactor(combobox-web): make the selector store observable with mobx — every selector and provider annotated with makeObservable; the setState({}) trigger kept so this commit is a pure store change.
3. refactor(combobox-web): replace the forced re-render with a snapshot hook — useObservableSnapshot (useSyncExternalStore over a MobX reaction) subscribed to the search term.
4. docs(combobox-web): record migration step 1 as an openspec change — openspec/changes/combobox-mobx-store.
5. feat(combobox-web): add the combobox core, its container and view model — src/core/: the generic envelope, the combobox ViewModel contract, and the composition root in the style of datagrid-web and gallery-web: comboboxConfig (static properties read once, including the selector family), ComboboxGateProps (the Mendix values the Core derives from), CB_TOKENS, ComboboxContainer + createComboboxContainer, FilterInputEventService (the debounced filter-input action as a setup component, disposed on unmount), ComboboxCore (injected with selector, config, gate and the service; cached slices, bound commands) and useComboboxCore. SingleSelection and MultiSelection read the snapshot only; the downshift hooks take the Core. getSelector, useGetSelector and the hand-built preview core props are gone; the editor preview uses createPreviewContainer. Specs: 19 Core and hook tests, 12 container tests, 6 service tests.
6. refactor(combobox-web): drop the deprecated search-term callback — onAfterSearchTermChange and its three tests.
7. docs(combobox-web): record migration step 2 as an openspec change — openspec/changes/combobox-viewmodel, status lines and the composition-root decision in the design docs.

Deferred on purpose, listed in the step 2 change: open state, input value and highlight stay with downshift until step 3 (Behavior), which also replaces the selectors' updateProps with derivations over the gate; option content is still rendered by the captions providers until step 4 (classes and parts), so the editor preview overrides survive untouched. brandi-react is not added: views read a plain snapshot and never resolve from the container.

Verification: pnpm exec tsc --noEmit clean; pnpm run test 18 suites, 213 tests, 5 snapshots unchanged, no -u; lint 0 errors; pnpm run build OK. E2E has not run: the build workflow only triggers for PRs targeting main, so it runs once #2499 merges and this branch is rebased.