refactor(quickadapter): rename known_at_index to known_at_lookahead (#96)

refactor(quickadapter): rename known_at_index to known_at_lookahead (#96)

PR #95 retained the historical column name `<label>_known_at_index` for what is now a per-row label lookahead in candles, to keep that hotfix strictly minimal. This PR converges the column suffix, the helper, the dataclass field, the static method, and the per-call-site locals onto `_known_at_lookahead`, with a retro-compat alias on the only externally-named public helper (`label_known_at_column_name = label_known_at_lookahead_column_name`).

The auxiliary `<label>_known_at_*` column is regenerated on every training run inside `set_freqai_targets`; FreqAI persists only the fitted model and `extra_returns_per_train`, never auxiliary dataframe columns -- the rename invalidates no on-disk artifact.

Reviewed by three parallel Oracle passes (math + claims-coherence; Python state-of-the-art + harmonization; documentation + terminology + PR-description coherence), each citing upstream evidence from `freqtrade/freqai/freqai_interface.py`, `data_kitchen.py`, and `data_drawer.py`. Consensus fixes were applied: README `causal_mode` formula symbol bound to the column token (`row-wise max(<label>_known_at_lookahead)`) to colocate definition with usage.

The two causal-guard local variable pairs were also harmonized to the local `train_<noun>` family (`train_known_at_lookahead`, `train_known_at_position`) used by the surrounding `_make_*_datasets` methods.