Retry async candidate load after rate-limit window

[adamtheturtle]

Wait out the remaining rate-limit interval instead of returning immediately. Closes #136

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Note

Low Risk
Small lifecycle/timing change in candidate loading; generation tokens still gate stale results, so risk is limited to brief delay/retry behavior.

Overview
When the palette is re-presented inside the 50ms load spacing window, the async candidate .task now sleeps for the remaining delay and retries instead of bailing out and never loading.

CandidateLoadGeneration.delayUntilNextLoadAllowed() reports that remaining interval. During the wait, isLoading is cleared so retained results stay visible without a stuck spinner. Generation tokens still reject cancelled or stale providers.

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