Stage 2 currently reads and writes calibration_package.pkl through loose dictionaries in several places. This step adds a typed payload boundary for the package pickle, moves package summary/checksum access behind reader/writer helpers, and writes calibration_package_meta.json from typed payload and contract material.

Scope:

• Add typed Stage 2 package payload reader/writer APIs.
• Preserve legacy pickle dictionary compatibility.
• Route contract summaries and remote package sidecar generation through the payload boundary.
• Add focused unit coverage for round trips, missing keys, legacy geography compatibility, summary parity, and remote sidecar generation.