GTK's symbolic icon recoloring engine injects CSS rules (fill: currentColor !important)
into symbolic SVGs. SVGs that rely on stroke outlines and fill="none" get their fills
overridden by currentColor, causing icons like New Tab and Task Board to render as
opaque solid blocks or silhouettes on systems enforcing strict symbolic recoloring.

Convert all stroke-based symbolic icons (board, tab-new, changes, terminal,
search, settings, dev.signaltty.gui-symbolic) to pure filled compound paths using
fill-rule="evenodd" with hollow inner cutouts. Add resource inspection in
app_tests to prevent future stroke-based regressions.

Co-authored-by: Lucenx9 <185146821+Lucenx9@users.noreply.github.com>