Replace the salvaged source-text regex test with a jsdom render of the
room workspace: styles.css is injected as a real <style> sheet and the
assertion reads getComputedStyle() of the <code> the shell renderer emits
inside a room body. A regex over the CSS source passes for any rule that
mentions the tokens, whether or not its selector reaches the room's DOM;
the cascade test is red when the room-owned selector is missing and green
only when it actually matches (verified by swapping styles.css to
origin/main).
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