fix: preserve calendar declarations across ingestion and updates #97

Problem

DataManager.load/import_data still use calendar=None for omission and write version-1 null metadata. A caller explicitly disabling calendars therefore gets a stored declaration that inherits NYSE downstream. Incremental gap detection also ignores the stored feed calendar and uses only an asset-class crypto hint, so a disabled schedule can still cause imputation.

Required behavior

Carry canonical calendar/FeedSpec declarations through load, import, initial update and incremental update. Explicit selection overrides declared feed/stored metadata, then NYSE. Preserve omitted versus explicit None and the temporal version; do not automatically relabel legacy payloads unless the caller explicitly overrides them. Persist timezone/timestamp/grid declarations that Data's Metadata supports. Validate incompatible declarations/calendars before publication. Exchange remains descriptive metadata, not an implicit selector.

Gap classification receives the selected calendar and declared semantics, not only the asset class. None disables schedule-derived detection and cannot manufacture rows from timestamp spacing even when the legacy fill flag is enabled. Missing-data acquisition and imputation remain separate. The requested frequency/bar type must agree with declared feed metadata, rather than silently storing contradictory meanings.

This depends on session/Metadata PR94; review the ingestion diff separately and require full main-target CI/docs/package checks after that prerequisite lands. Unsupported intraday-grid/daily event interpretations remain explicit, not a claim of all calendar conformance. Preserve published0.2.1; no release/deploy or heavy local suite during Backtest's timing window. AI-assisted implementation under maintainer direction.