CANDLE_PROVIDER resolves a symbol's candles per-granularity (1d/1wk) from the
shared cache, with a reversible fold-in of the freshest live quote so a
mid-session evaluation sees the current price. Pure fold logic + todayIso are
unit-tested; CacheCandleProvider is a thin cache-backed shim. Evaluators use
this seam instead of cache.get inline, so a future realtime/replay source can
slot in without touching slot logic.
