feat(quickadapter)!: add causal label split foundation (#78)

feat(quickadapter)!: add causal label split foundation (#78)

Causal split guards on QuickAdapter training. Default causal mode
rejects `data_split_parameters.shuffle=true`,
`feature_parameters.shuffle_after_split=true`, and
`feature_parameters.reverse_train_test_order=true`.

- `feature_parameters.causal_mode` (default `true`): guard toggle.
  `false` is deprecated.
- `feature_parameters.label_horizon_candles` (default
  `label_period_candles`): candles after a label row before its label
  is considered known by causal split guards. Fallback chain
  `label_horizon_candles` -> `label_period_candles` -> `1`.
- `<label>_known_at_index` columns expose `LabelData.known_at_index`
  per-row; multi-label boundary via element-wise max across present
  columns.
- `timeseries_split` `gap` auto-set from `label_horizon_candles` under
  causal mode; explicit `gap < label_horizon_candles` rejected.
- Persisted Optuna `label` best-params JSON has shape
  `{schema_version, params}`
  (`_OPTUNA_LABEL_BEST_PARAMS_SCHEMA_VERSION = 2`). Unversioned files
  identified by shape; version-mismatched files emit distinct
  "missing" vs "incompatible" warnings.
- `_label_aux_column_name` shared sigil-stripping helper backs
  `label_weight_column_name` and `label_known_at_column_name`;
  uniform collision guard against `&`/`%` and empty stem.
- `QuickAdapterRegressorV3.version = 3.12.0`.

BREAKING CHANGE: `feature_parameters.causal_mode` defaults to `true`.
Configs with `data_split_parameters.shuffle=true`,
`feature_parameters.shuffle_after_split=true`, or
`feature_parameters.reverse_train_test_order=true` raise at training
time.