Frames-native reconciliation validation guards

[Polichinel]

Epic: #31 · Depends on: #33

Background

The original ReconciliationModule.__init__ validates fail-loud before doing any work: dataset types, temporal alignment (same number of time steps + same time unit + exact overlap), target intersection (common prediction columns), and valid countries (present in both cm and pgm). The mirrored red-team tests live in views-reporting/tests/test_reconciliation_module.py (TestReconciliationTypeValidation, TestReconciliationTemporalValidation).

Work

• Port these guards frames-native (compare cm/pgm indices + target sets) with the same fail-loud ValueError intents and messages.
• Keep them as small, independently testable helpers (SRP) — not buried inside the orchestration class.

Acceptance criteria

• Mismatched time-step count / time unit / non-overlapping times / no-common-targets each raise a clear ValueError.
• The valid-country intersection is computed and exposed for the orchestrator.

Validation

• Red-team unit tests mirroring views-reporting's validation tests.