fix: preserve calendar declarations and instant boundaries in session APIs #93

Reproduced session-boundary problems

On55b86e250fd6279222876c5a1e4c20ae1469d7fa, DataManager.assign_sessions on a real SyntheticProvider frame with calendar=None raises Must provide either exchange or calendar rather than disabling schedules. Omission also fails instead of NYSE. SessionAssigner('24/7').assign_sessions on continuous-provider hourly bars at July7 00:00 and01:00 UTC, bar_frequency='intraday', assigns null at midnight instead of the new day's session because the forward-close join selects the previous closed interval. DataManager without storage also ignores complete_sessions(fill_gaps=False), unlike its storage-backed path.

Required contract

Canonical calendar constructor/API selection uses shared FeedSpec resolution: explicit, declared input/feed metadata, then NYSE. Explicit None returns observations unchanged without a manufactured session column or imputation. Keep positional names and calendar_name/exchange selector arguments as compatibility aliases; contradictory explicit selectors fail clearly. Metadata exchange identity alone is not a selector.

Version-2 Data Metadata must preserve omitted versus explicit-null calendar through JSON/copy and shared FeedSpec projection; retain version-1 legacy null inheritance. Delegate normalization/serialization to shared Specs, not a separate resolver. Propagate declared timestamp semantics/timezone through DataManager and low-level session adapters.

Session labels preserve civil dates. Explicit event_time/bar_close declarations override daily-frequency and midnight heuristics. Event intervals are [open,close), including the exact start of a continuous session; close-labeled bars use (open,close]. Apply matching break boundaries. Retain existing null/raise/drop outside-session policy. Naive instants retain the existing UTC interpretation unless the feed explicitly declares a timezone; ambiguous/nonexistent local times fail. Daily session labels are not event instants.

Session completion remains an explicit minute-grid operation. fill_gaps=False performs assignment without expansion, independently of storage configuration. Calendar=None performs no expansion. Existing minute completion supports open-labeled event bars only; unsupported explicit close-label/session-label declarations fail clearly rather than being silently used as minute instants. Intraday gap-grid classification and futures roll scheduling remain separate accepted repairs.

Use real provider/public-facade and persisted Metadata regressions, exact-source failing-base/passing-fix evidence and hosted full gates. Keep the workstation reservation: light focused single-thread local checks only. No release/deployment, new agents or changes to published0.2.1. AI-assisted implementation under maintainer direction.