Phase 4 Step 8: Accessibility tree

[thomasnemer]

Parent: #41

Goal

Build an accessibility tree parallel to the DOM and expose it to platform assistive technologies via AccessKit.

Prerequisites

• Step 7 (Shadow DOM) — the composed tree is what the accessibility tree represents

Implementation

• Build accessibility tree parallel to DOM: compute role, name, state, and relationships from ARIA attributes and HTML semantics
• Platform bridge via accesskit crate (AT-SPI2 on Linux, NSAccessibility on macOS, UIA on Windows)
• Integrate accesskit_winit with the existing winit event loop in ie-shell
• Incremental updates: push a11y tree changes when DOM mutates
• Map HTML elements to appropriate ARIA roles by default (implicit semantics)
• New dependencies: accesskit, accesskit_winit

Tests

• Accessibility tree construction tests (correct roles, names, states)
• ARIA attribute mapping tests
• Implicit role tests for HTML elements
• Incremental update tests (DOM change → a11y tree update)
• Integration test with AccessKit adapter

Acceptance Criteria

• Accessibility tree is built from the composed DOM tree
• ARIA roles, names, and states are computed correctly
• Platform assistive technologies can read the tree via AccessKit
• Tree updates incrementally when the DOM changes