refactor(quickadapter): disambiguate label_period_candles vs label_horizon_candles

refactor(quickadapter): disambiguate label_period_candles vs label_horizon_candles

Three distinct issues addressed together for terminology coherence
and operator visibility.

(1) README: replace "NATR horizon" by "NATR period" in the four rows
documenting `label_period_candles` / `min_label_period_candles` /
`max_label_period_candles` / `label_candles_step`. The noun
"horizon" was also the noun in `label_horizon_candles`, which has
the opposite temporal direction (lookahead causal-split guard vs
lookback NATR period). "NATR period" matches both the tunable name
(`label_period_candles`) and the underlying API
(`ta.NATR(..., timeperiod=label_period_candles)`).

(2) `QuickAdapterV3.set_freqai_targets`: rename the local
`label_period` (a `datetime.timedelta` spanning
`len(dataframe) * timeframe_minutes`) to `series_duration`, and
update the two log labels (`label_period: 3 days, 12:00:00` →
`series_duration: 3 days, 12:00:00`). The previous name collided
with `label_period_candles` (an int candle count) at the operator
log level; `series_duration` matches the sibling `series_length`
(an int) declared two lines below.

(3) `QuickAdapterRegressorV3.__init__` startup dump: add
`label_horizon_candles` to the "Label Parameters:" section (both
HPO-enabled and HPO-disabled branches). The parameter is a
training-time data-split lookahead (causal split guards in
`_make_train_test_split_datasets` / `_make_timeseries_split_datasets`),
NOT an HPO hyperparameter — it is neither sampled
(`trial.suggest_int`) nor bounded (`min/max_*`) nor an HPO input to
`label_objective`. Placing it in "Label Parameters:" (runtime
values) rather than "Label Hyperparameters:" (HPO config) matches
its actual semantics. In the HPO-on branch the global value is
emitted before the per-pair loop to visually distinguish it; in the
HPO-off branch it trails the existing global lines so the natural
order is `label_period_candles` → `label_natr_multiplier` →
`label_horizon_candles` (the latter falls back to the former when
unset).