Unify App process scope and foreground-window semantics - #436

Motivation

• Make App a consistent process-scoped abstraction across platforms so callers see one logical application per process while still exposing every native registration where it matters.
• Fix foreground-window tagging to use the provider's true foreground identity and avoid misclassifying macOS key panels or Linux multi-registration cases.

Description

• Collapse same-PID native registrations into a single public App view by deduplicating list_apps() results (process-scoped enumeration) and adding fn app_roots(&self, app: &ElementData) -> Result<Vec<ElementData>> to Provider so backends can expose every native root for a process.
• Make App::windows_with / App::children / App::tree traverse all native roots for a process, merge windows by stable identity, and avoid duplicate listings; add dedup_processes and update tag_focused to use the provider's focused_app() identity rather than a pid-only heuristic.
• Extend Locator with process-scoped resolution (new_for_app) and support for searching across multiple native roots with stable-identity deduplication to preserve doc-order and :nth semantics.
• Linux (AT-SPI): add find_apps_by_pid and app_roots implementations and update subscribe_for_pid to install D-Bus match rules for every AT-SPI registration owned by the process and to preserve the originating sender bus when materializing events; EventContext now accepts multiple sender_buses and dispatches only signals from those senders.
• macOS (AX): change active mapping from AXMain to the frontmost application's focused (key) window by reading AXFocusedApplication/AXFocusedWindow to correctly mark window active when key differs from main.
• Documentation: update the Linux accessibility quirks page to describe the migration from registration-scoped to process-scoped App and how callers should select windows/registrations now.

Testing

• Ran cargo test -p xa11y-core --lib, which passed all unit tests in xa11y-core (283 tests, all OK).
• Ran RUSTFLAGS=-Dwarnings cargo check -p xa11y-core -p xa11y-linux -p xa11y-macos, which succeeded (no warnings-as-errors).
• Ran cargo xtask check-macos-ffi, which passed the macOS FFI exception-safety check.
• Ran cargo xtask fmt --check, which passed formatting checks.
• Ran cargo xtask check (workspace pre-PR checks); the tooling completed its linting, bindings-parity, format and many checks successfully, but full workspace test/linking failed where the environment lacks system dependencies: the build/link stage for xa11y-linux/fuzz targets failed due to missing libxkbcommon (linker error), and cargo xtask test-integ could not run because the runner lacks Xvfb/AT-SPI daemons and other system fixtures; these are environmental, not functional, failures in CI-like harnesses.
• cargo xtask lint-docs could not complete because vale is not installed in the environment; prose linting should be verified in CI or locally with vale available.

If you want, I can open follow-ups to (1) add CI notes about the new Provider::app_roots contract for downstream providers, and (2) prepare a changelog/migration note for bindings/docs that call out the behavioral change from registration-scoped to process-scoped App objects.

Codex Task