[P1][WS-04] Command palette, semantic selection and accessibility

Programme: #1
Epic: #16

## Objective
Make keyboard, direct manipulation, accessibility and semantic selection equal first-class interaction modes alongside voice/chat.

## Scope
- Global command palette over workspaces, objects, actions, files, tasks and providers.
- Stable semantic selection model for rows/cards/entities/evidence/files/panels.
- Keyboard navigation, focus management, shortcuts and discoverability.
- IME/composition, screen-reader semantics, zoom/density, high-contrast/reduced-motion hooks and localization-ready text.
- Context handoff so commands such as “explain this” resolve explicit selection IDs.
- Deterministic global/local search over already indexed material.

## Product rules
- Core workflows must not require speech or inference.
- Selection/context is explicit and scoped; no implicit unrestricted desktop capture.
- Generated layouts must preserve accessible names/roles/order.

## Acceptance criteria
- [ ] Core workspace/research/browser/task flows complete using keyboard and assistive technology.
- [ ] Selection survives view refresh where the underlying object still exists.
- [ ] “this/these/current” commands resolve to explicit semantic targets and fail when ambiguous.
- [ ] IME and text composition work correctly in trusted and embedded browser surfaces.
- [ ] Zoom/density changes do not break trusted controls or action visibility.
- [ ] Accessibility regression tests cover generated component layouts.

## Dependencies
- WS-01

**First phase:** P1
**Maturity target:** P5
**Owner:** workspace-ui-data

## Task issues
- [ ] #336 `WS-04.T01` — Define command descriptors and routing
- [ ] #337 `WS-04.T02` — Implement explicit semantic selection
- [ ] #338 `WS-04.T03` — Build command palette and local search
- [ ] #339 `WS-04.T04` — Implement focus and input composition
- [ ] #340 `WS-04.T05` — Expose accessible component semantics
- [ ] #341 `WS-04.T06` — Implement zoom, density and preference controls
- [ ] #342 `WS-04.T07` — Integrate contextual voice targets
- [ ] #343 `WS-04.T08` — Run keyboard and assistive-technology conformance

Start by reading the dependency WS-01 and the task issues #336–#343, then identify which task has a defined entry point. Use the listed acceptance criteria to scope the work: keyboard and assistive-technology workflows, persistent semantic selection, unambiguous contextual commands, correct IME and zoom behavior, and accessibility regression coverage.

Written by the indexing model from the issue text.