Task: Implement the Icon Theme Locator feature

TASK-DESCRIPTION

• Create sources/desktop/icon_theme_locator.h.
• Create sources/desktop/icon_theme_locator.cpp.
• Create the portable test tests/icon_theme_locator_test.cpp.
• Create the test runner tests/12-icon-theme-locator.sh.
• Implement the requirements in prompts/features/07-icon-theme-locator.md.
• Run make test from the repository root.

TASK-OUTPUT

• Report the created files and the result of make test.

TASK-CONTEXT

• Feature requirements: prompts/features/07-icon-theme-locator.md.
• Icon search rules: documents/12-desktop-loading-and-provenance.md section 3.
• The test builds its own XDG tree under a temporary directory.

TASK-FILES

• sources/desktop/icon_theme_locator.h — new
• sources/desktop/icon_theme_locator.cpp — new
• tests/icon_theme_locator_test.cpp — new
• tests/12-icon-theme-locator.sh — new