fix: adopt shared calendar contract in session-aware labeling #94

Calendar-aware labeling must preserve shared calendar selection and row-mode semantics

Affected source: main0e7865273cd462c507de02a9d3b64de7c6cbb099. Distinct from fixed-intraday feature ownership in closed issue76/PR90 and authoritative EquityCalendar helper issue91.

Public reproduction
from datetime import datetime
import polars as pl
from ml4t.engineer.config import LabelingConfig
from ml4t.engineer.labeling import calendar_aware_labels
data = pl.DataFrame({'timestamp': [datetime(2025,7,4,12), datetime(2025,7,7,12)], 'close': [100.,101.]})
calendar_aware_labels(data, LabelingConfig.triple_barrier(max_holding_period=1), calendar=None)

Observed: TypeError requiring callable calendar protocol methods. Omitting calendar also fails because it is mandatory. No shared FeedSpec path is accepted, and LabelingConfig cannot preserve a calendar declaration through JSON/YAML. Existing standalone numerical labeling kernels are separate from this calendar-aware wrapper.

Expected next-minor contract
Use canonical calendar and the shared Specs resolver: explicit argument > explicitly declared config selector > feed metadata > NYSE. Preserve omitted versus explicitnull through copied/serialized configs. calendar=None labels observation rows in input order without loading a provider or requiring timestamps; a named24/7 schedule remains calendar-aware. Keep legacy custom protocol calendars and explicit auto-gap mode as documented compatibility paths, never as proof of completeness. Consume existing FeedSpec column, timezone and timestamp declarations without a new wire format. Reject session-label intraday inputs and unsupported window units explicitly. Observation counts, validated expected bars, elapsed durations and session counts must remain distinct; expected-bar mode must validate the grid before labeling. Use declared timezone for naive input; do not guess holidays/frequency from data spacing.

Acceptance
Real public price->label and JSON/YAML->label workflows fail on the base and pass with the fix. Include row invariance across timestamp spacing/order, interleaved assets, absent timestamps, metadata null/name/omission precedence, exchange closures, declared naive timezone, invalid settings atomicity and expected-grid outage rejection. Run light focused checks locally and full hosted supported source/wheel/docs/package gates before ordinary merge. Preserve standalone legacy kernel behavior and document its boundary; do not claim full harmonization or authorize a release/deployment.

AI assistance: Codex reproduced the public failure, reviewed shared Specs contracts and prepared the focused consumer integration and executable evidence. Repository owner remains accountable for the source and results.