feat(cdp): implement Accessibility domain (getFullAXTree)

Implements the CDP Accessibility domain with getFullAXTree support,
enabling AI agent tools (agent-browser, Playwright MCP) that rely on
Accessibility.getFullAXTree to work with Obscura.

Changes:
- Add crates/obscura-cdp/src/domains/accessibility.rs (~380 lines)
  - Accessibility.enable: returns {}
  - Accessibility.getFullAXTree: walks DOM tree, produces AXNode array
  - Role mapping: 50+ HTML tags → ARIA roles, respects explicit role attr
  - Name computation: aria-label → aria-labelledby → alt → title → placeholder
  - Properties: focusable, editable, checked, disabled, level, required, multiline
- Update mod.rs: register accessibility module
- Update dispatch.rs: route Accessibility domain to real handler

Tested with example.com (18 nodes), httpbin.org/html (16 nodes),
news.ycombinator.com (1308 nodes). Zero structure errors.

Closes #12

crates/obscura-cdp/src/domains/mod.rs

@@ -7,4 +7,5 @@ pub mod network;
 pub mod fetch;
 pub mod input;
 pub mod storage;
+pub mod accessibility;
 pub mod lp;