feat: add page.accessibility CDP domain API

[myabc]

Problem

Reading an element's computed accessibility properties (role, name, description, ARIA state) under Ferrum means reaching past the public API into raw CDP:

page.command("Accessibility.getPartialAXTree",
  nodeId: node.node_id, fetchRelatives: false)["nodes"]

This leaks the CDP wire shape into callers, is undiscoverable, and leaves no home for the rest of the Accessibility domain.

Fix

Add a first-class page.accessibility domain object, mirroring Ferrum::Network:

• node_for, partial_tree, snapshot, query, root, enable/disable. Node-scoped commands run on the node's own page, so they resolve correctly across same-process iframes.
• Ferrum::Accessibility::AXNode value object wraps a CDP AXNode: role, name, description, value, properties, ignored?, ignored_reasons, node_id, backend_dom_node_id, child_ids, to_h. Params are deep-frozen; value is untyped since CDP AXValue.value may be a string, number, or boolean.
• Ferrum::Node#axnode convenience, mirroring #computed_style.

Query commands work without Accessibility.enable (that governs live events only); enable/disable are thin wrappers. root uses getFullAXTree(depth: 1) and query falls back to the document root node id, because CDP getRootAXNode/queryAXTree require those respectively.

Testing

RBS signatures plus unit and browser-driven specs, including subtree-scoping proof and same-process iframe coverage for node_for, partial_tree, and query. CHANGELOG updated.

Downstream validation

🤖 Validated against a real consumer: capybara_accessible_selectors#157 (Cuprite driver support) resolves computed name/role/description from the accessibility tree.

Swapping its hand-rolled Accessibility.getPartialAXTree + ignored-node filtering + hash-digging for node.axnode&.name / .role / .description removed the raw-CDP coupling entirely. Full suite green on Cuprite — 1813 examples, 0 failures.

Before:

nodes = node.page.command("Accessibility.getPartialAXTree",
                          nodeId: node.node_id, fetchRelatives: false)["nodes"]
Array(nodes).find { |n| n["ignored"] == false }&.dig("name", "value")

After:

node.axnode&.name

[Copilot]

Pull request overview

This PR introduces a first-class page.accessibility domain API for interacting with Chrome DevTools Protocol (CDP) Accessibility domain data through Ferrum, plus a Node#axnode convenience accessor and accompanying RBS + specs.

Changes:

• Add Ferrum::Accessibility (domain wrapper) and Ferrum::Accessibility::AXNode (value object) to encapsulate CDP Accessibility queries.
• Expose the API via Page#accessibility, Browser#accessibility delegation, and Node#axnode.
• Add unit + browser-driven specs, test fixture pages, and update RBS + CHANGELOG.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
spec/unit/accessibility/ax_node_spec.rb	Adds unit coverage for AXNode behavior and immutability.
spec/support/views/accessibility.erb	Adds a fixture page for accessibility tree querying tests.
spec/support/views/accessibility_iframe.erb	Adds an iframe fixture page for cross-frame accessibility tests.
spec/support/views/accessibility_iframe_child.erb	Adds iframe child content for frame-resident node accessibility tests.
spec/node_spec.rb	Adds integration coverage for Node#axnode.
spec/accessibility_spec.rb	Adds browser-driven coverage for page.accessibility APIs (node_for/partial_tree/query/root/value, incl. iframe).
sig/ferrum/page.rbs	Exposes Page#accessibility in RBS.
sig/ferrum/node.rbs	Exposes Node#axnode in RBS.
sig/ferrum/accessibility/ax_node.rbs	Adds RBS signature for Accessibility::AXNode.
sig/ferrum/accessibility.rbs	Adds RBS signature for Ferrum::Accessibility.
lib/ferrum/page.rb	Wires Accessibility into Page initialization and adds accessor docs.
lib/ferrum/node.rb	Adds Node#axnode convenience method.
lib/ferrum/browser.rb	Delegates accessibility from Browser to the current page.
lib/ferrum/accessibility/ax_node.rb	Implements AXNode value object and deep-freeze.
lib/ferrum/accessibility.rb	Implements the Accessibility domain wrapper methods.
CHANGELOG.md	Documents the new page.accessibility API and Node#axnode.

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