fix(calendar)!: use UTC daily crypto sessions and maintenance - #103

Closes #100.

Use the existing authoritative PandasMarketCalendar 24/7 adapter for CryptoCalendar. Enumerate one daily UTC session open rather than treating every hour as a session. Normalize membership and annotations to UTC, subtract explicit half-open maintenance windows including overnight windows, and return strict adjacent session boundaries rather than arbitrary immediate instants or intraday maintenance reopens. Canonical calendar='24/7' and the existing CRYPTO_24_7 alias select the continuous schedule; None belongs to calendar-free consumers and is rejected on a concrete schedule object.

This intentionally changes legacy helper outputs on the next-minor 0.2 development line. It is not part of the historical 0.1.7 patch. Existing maintenance constructor arguments/name remain; invalid windows fail before constructing a misleading schedule. The calendars extra is now required rather than silently manufacturing a schedule. The existing feature guide explains units, boundary/maintenance/timezone changes and row-mode migration. No new wire format or annualization rule.

Evidence at f61ae28, tree ac9e5f0be5888692d5591ffa0e05d63d08d2a677:

• New public helper regressions on unchanged base9364c86: 29 failed/1 passed in 0.48s. The same cases against a normally installed b95fb29 wheel with the unchanged producer: 29 failed/1 passed in 0.34s. These exercise actual session enumeration, UTC/Tokyo/New York membership, overnight duration, original-row annotations and boundary calls, not copied schedule logic.
• Initial repair passed 28/30; exact-midnight previous-close cases exposed insufficient backward lookup. Fixed the lookup, retaining both tests.
• Final new/existing helper suite: 90 passed in 4.61s. Normal installed wheel: 90 passed on Python3.12/3.13/3.14 in 4.22s/8.03s/4.77s. Actual installed origin and changed producer bytes match the wheel. Python3.12 dependency consistency passes all31packages.
• Full clean source suite: 4414 passed, 57 deselected, 17 warnings in 93.08s. Whole Ruff/format/ty, all-file hooks and plain commit hooks pass.
• Executable documentation workflows, strict MkDocs2.93s, wheel/sdist builds and Twine metadata checks pass. Candidate manifest binds this source/tree/version and both hashes.
• Wheel SHA256 a69511f27369e9154f5f9a290ff64f92e4c32b77a1a359e8b3e9aeb3ea2b7db6. Sdist SHA256 1e70078c1ff5399fb2215c0c03824d7c5ec0476efcba6b48ea2aa47c6e291cbc.

Current-head supported hosted gates remain required before ordinary merge. This branch still honestly uses the existing immutable Specs qualification dependency; published dependency adoption is independently qualified in PR102. Remaining daily feature grids and session/elapsed consumers are not claimed complete by this helper repair. No release/tag/docs deployment.

AI assistance: Codex reproduced the actual source/installed workflows, implemented and reviewed the focused repair and migration text, and executed the listed checks. No independent human review is claimed.