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/dispatch.rs

@@ -135,8 +135,9 @@ pub async fn dispatch(req: &CdpRequest, ctx: &mut CdpContext) -> CdpResponse {
         "Input" => domains::input::handle(method, &req.params, ctx, &req.session_id).await,
         "Storage" => domains::storage::handle(method, &req.params, ctx, &req.session_id).await,
         "LP" => domains::lp::handle(method, &req.params, ctx, &req.session_id).await,
+        "Accessibility" => domains::accessibility::handle(method, &req.params, ctx, &req.session_id).await,
         "Emulation" | "Log" | "Performance" | "Security" | "CSS"
-        | "Accessibility" | "ServiceWorker" | "Inspector"
+        | "ServiceWorker" | "Inspector"
         | "Debugger" | "Profiler" | "HeapProfiler" | "Overlay" => {
             Ok(json!({}))
         }