fix: plan daily backfill ranges from declared exchange sessions #91

Problem and reproduction

At Data main55b86e250fd6279222876c5a1e4c20ae1469d7fa, SyntheticProvider.fetch_ohlcv('SPY','2025-07-03','2025-07-07','daily'), excluding the known July4 closure, returns July3 and July7 bars. update_manager.GapDetector(exclude_weekends=True).detect_gaps incorrectly requests July4 as a backfill gap. PR90 repaired the separate gap utility, not this range planner. Its storage-range boundary and empty-dataset paths also count elapsed days without the selected exchange schedule.

Required behavior and compatibility

Use shared FeedSpec declaration resolution: explicit calendar, declared feed/stored metadata, then NYSE. None disables schedule-based range classification; 24/7 still detects real missing days. Preserve the explicit legacy exclude_weekends=False choice as a continuous-calendar compatibility hint, lower priority than a canonical selection or declaration. Omitted configuration now follows NYSE. exclude_weekends=True uses NYSE sessions, including holidays.

Plan daily gaps from authoritative session labels over the requested inclusive date range, including an entirely absent dataset/session and leading/trailing gaps. Daily returned bounds are UTC midnight session labels; the legacy size_days key counts missing selected sessions, not elapsed closed days. Date-label bounds must match actual provider rows during backfill even when those rows use non-midnight daily timestamps. Group consecutive missing schedule positions into one acquisition range; classification is not imputation and must not create rows or mutate storage.

Retain existing intraday/weekly compatibility paths without claiming that they are calendar-aware. Intraday grid semantics, DataManager session mapping and futures roll scheduling remain separate repairs. No release/deploy; published0.2.1 stays unchanged. AI-assisted implementation and verification under maintainer direction. Full verification uses hosted workflows while Backtest has reserved the workstation timing window.