When a spell is queued in the 400ms window before the current cast ends,
render its icon dimmed (0.8 alpha) to the right of the cast bar progress,
with a "Queued: <name>" tooltip. The progress bar shrinks to accommodate
the icon when one is present.
Also exposes getQueuedSpellId() as a public const accessor on GameHandler
so the UI can observe the spell queue state without friend access.
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