Darwin: fix ARM64 build and runtime compatibility issues

[moreaki]

Summary

Add the minimal Darwin/Qt build compatibility fixes needed for the macOS/Arm test tree.

The changes cover:

• ignore Homebrew's include/c++ fallback when probing iostream on Darwin;
• treat an explicit Darwin --with-qt-path as a normal Qt framework root instead of VirtualBox's private libQt6CoreVBox tool package;
• derive Qt framework header and -F paths from the detected QtCore.framework;
• cast Carbon hotkey Unicode constants before constructing QChar, matching Qt 6's stricter overload set;
• fix two small Darwin warning issues in DNS and power-service code.

Why

These are the smallest supporting fixes needed to keep the external macOS/Arm build usable for the Windows-on-Arm HiDPI and graphics-throughput work tracked in #742.

Compatibility note

The Qt path change is Darwin-only. Non-Darwin builds keep the existing tool-package path. Darwin builds without an explicit custom Qt root also keep the existing tool lookup. The custom-root path now only changes behavior when --with-qt-path points at a framework-based Qt installation.

The iostream probe change is also Darwin-only and only rejects Homebrew's include/c++ fallback for that standard C++ header.

Validation

• Applied independently to origin/main.
• git diff --check passes.
• python3 -m py_compile configure.py passes.
• A Qt 6.11.1/Carbon syntax probe confirms QChar(kShiftUnicode) is rejected while QChar(static_cast<ushort>(kShiftUnicode)) compiles.
• A direct -F /opt/homebrew/opt/qt/Frameworks syntax probe confirms <QtCore/QtGlobal> is usable through the framework path.
• A reduced configure probe with --only-qt --disable-kbuild --disable-xcode --with-qt-path=/opt/homebrew/opt/qt reaches the Darwin framework path and reports Using framework at '/opt/homebrew/opt/qt/Frameworks/QtCore.framework'.

Refs #742.

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