fix: distinguish continuous exchange sessions from hourly crypto observations #100

Problem and accepted requirement

Legacy core CryptoCalendar.sessions_between describes and enumerates every hour as a session. Exchange-session counts and hourly observations are different units. A named continuous calendar also differs from calendar-free row execution. The shared time contract requires explicit, consistent units and canonical calendar selection across public helper and consumer paths.

Source evidence

src/ml4t/engineer/core/calendars/crypto.py sessions_between advances by timedelta(hours=1); the class also exposes explicit maintenance and timezone settings. This is current source evidence, not a new executed end-to-end reproduction.

Scope and acceptance

• Reproduce the public helper behavior before changing it and retain exact base/fix evidence. A continuous daily schedule must enumerate actual daily session starts, not manufacture hourly sessions.
• Reuse authoritative named schedule adapters, preserve the explicit calendar argument/metadata/NYSE resolution contract at consumers, and reject calendar=None where a concrete named calendar object cannot represent row mode. Do not reinterpret named24/7 as row mode.
• Resolve membership, next-open/previous-close, duration, session enumeration and timestamp/session-label annotations consistently, including timezone conversion, daily boundaries and explicit maintenance behavior.
• Preserve compatible legacy constructor arguments where possible; surface output-changing behavior and its next-minor migration. Do not silently discard explicit maintenance settings or invent a new serialized calendar format.
• Keep annualization separate from calendar membership and session enumeration.
• Qualify public helper/feature/label consumer workflows, relevant supported source/wheel checks and migration documentation. Do not claim complete calendar adoption from a helper-only unit test.

AI assistance

Codex reviewed the source and reconciled the accepted implementation requirement. Reproduction, repair and final qualification remain unexecuted for this focused item.