fix: plan and acquire daily backfills from declared sessions - #92

Closes #91

Outcome

The separate update/backfill range planner now uses authoritative daily exchange sessions with shared FeedSpec resolution: explicit selection, declared feed/stored metadata, then NYSE. Explicit None disables schedule classification; 24/7 preserves real weekend gaps. It covers leading/trailing ranges and entirely absent datasets, not just gaps between observations.

Daily range bounds are inclusive UTC-midnight session labels, and size_days/tolerance_days count missing selected sessions. Legacy exclude_weekends=False retains continuous days absent a canonical declaration; omission and True follow NYSE. Intraday/weekly legacy paths are not claimed repaired, and named declarations for those paths are rejected rather than ignored.

Backfill matches daily labels rather than midnight event bounds, so non-midnight provider bars are retained. It only acquires scheduled rows and cannot insert provider observations for closures inside a range. Declared label timezone survives storage's canonical UTC reads. Classification itself never fills or writes storage.

Reproduction and verification

• Original production base55b86e250fd6279222876c5a1e4c20ae1469d7fa, imported from an isolated git archive: five actual workflow regressions fail on phantom July4 gaps, closed-only storage boundaries, and closed-date provider rows being inserted during backfill. No new-keyword TypeError is used as evidence for those five defects.
• Regression-only6a90325: initial26 cases produce24 failures/2 controls passing.
• Current head ee48e05 (production repair93c043d): tests/test_calendar_backfill_ranges.py and tests/test_update_manager.py pass86 cases with ResourceWarning as error, POLARS_MAX_THREADS=1. New producer/storage cases cover both Flat and Hive, all-asset missing sessions, explicit/null/continuous metadata including a real weekday outage, per-call overrides, boundaries, timezone labels and invalid-input atomicity. Six relevant incremental-strategy checks also pass.
• Ruff lint/format, ty and commit hooks pass. Relevant incremental-strategy checks are also run locally. Full coverage/resource/platform/package/ecosystem and strict docs checks must pass in hosted CI before merge.
• No local full suite or benchmark is launched during Backtest's workstation timing reservation. No release or deployment; 0.2.1 remains unchanged. The immutable Specs source pin is unchanged.

Review and assistance

AI-assisted source, regressions and prose under maintainer direction. Owner reviewed schedule independence from observed dates, range units/bounds, declared-null propagation, storage/backend parity, timezone restoration, provider-row selection and legacy compatibility. Full harmonization still requires the separate intraday/session API and roll-calendar repairs.