[JFWooten4]

Summary

Implement opener-0114 (§2.4, Detect when another application becomes the default PDF handler).

• Add a read-only detect-pdf-handler-change utility that compares the macOS Launch Services PDF handler against PDF Magic Opener's installed bundle identifier.
• Reuse the PDF-specific registeredPDFHandlerBundleIdentifier() lookup already merged in PR #59, rather than duplicating the native handler-query logic or using the unrelated default web browser.
• Return distinct statuses: 0 when PDF Magic Opener remains default, 2 when another application has taken over (displaying its bundle ID), and 1 when the association cannot be determined. Re-read the association on every check and do not mistake an unknown handler for a change.
• Build and deploy the check as an optional CLI utility; document its usage, result codes, and how to restore the opener only if the user chooses. The detector does not run persistently in the background or change the PDF association.

Why

The installed set-pdf-default-handler.swift can register the opener and the merged identify-pdf-handler.swift can report the current PDF handler, but no existing command distinguishes opener still default from another application has become default. This focused change provides that distinction without silently reverting user customizations.

Verification

• Add synthetic macOS regression coverage for opener → Preview, opener → Adobe Reader, restoration, absent registration, and fresh lookup on each check.
• Compile and execute the detector tests within the existing required macos-build CI job; keep scanner-tests unchanged.
• Require both CI jobs to pass against the final PR head; confirm the PR is squash-merged and GitHub reports merged=true.

Scope and preservation

Only the PDF default-association detection is changed. There are no changes to ClamAV, quarantine, scanner caching, user-selected browsers, opener association registration, fallback behavior or user's PDF settings. No secret-bearing files were accessed. The isolated branch is pdf-handler-takeover. The available GitHub connection publishes as JFWooten4; no alternate identity or credentials were accessed.