docs: record that QString::arg does not collapse %% into %

printf does, and the habit transfers silently. In generated CSS it is
quietly destructive: a percentage written %% reaches the browser malformed,
and a browser drops that one declaration and renders the rest, so the pane
still paints and nothing looks broken. The 0.11.0 placeholder lost its
mask, its glow and both radial gradients this way.

Records the review lesson alongside it, which generalizes further than the
rule: a geometry probe endorsed that layout because it measured only
properties carrying no percentage. A probe that cannot see the thing that
breaks reports success forever.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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