fix: allow shadow-only forward observation receipts

[Pigbibi]

Summary

• Allow a forward-observation receipt to bind a Shadow-only policy without inventing Paper evidence.
• Keep receipt evidence modes an exact match for the frozen candidate policy.
• Align the Chinese runtime contract with the target-capability semantics introduced in fix: separate shadow and paper lifecycle evidence #424.

Safety

No strategy parameters, brokers, deployment targets, credentials, order path, or live authority changes.

Verification

• rtk pytest tests/test_forward_observation.py tests/test_forward_observation_receipt.py tests/test_paired_shadow_evidence.py
• git diff --check