fix(capture-core): reject duplicate JSON keys and non-finite numbers in load_state/sha_mapping and check seal.alg in check_seal #366

Summary

Three state-integrity boundary gaps exist in packages/agentrust-capture-core/src/agentrust_capture_core/:

1. Duplicate JSON keys and non-finite constants accepted in load_state (packages/agentrust-capture-core/src/agentrust_capture_core/state.py lines 65-80):

   • load_state(path) calls bare json.loads(path.read_text(encoding="utf-8")) without object_pairs_hook or parse_constant, accepting state/baseline files with duplicate JSON keys or NaN/Infinity constants instead of returning None (corrupt state).

2. allow_nan=True default in sha_mapping (packages/agentrust-capture-core/src/agentrust_capture_core/hashing.py lines 76-80):

   • sha_mapping(value) calls json.dumps(value, sort_keys=True, separators=(",", ":")) without allow_nan=False, allowing NaN and Infinity to serialize into non-standard JSON before hashing.

3. Unchecked seal["alg"] in check_seal (packages/agentrust-capture-core/src/agentrust_capture_core/seal.py lines 61-71):

   • attach_seal writes {"alg": "SHA-256", "digest": state_digest(snapshot), "sealed_at": ...}, but check_seal only checks isinstance(seal.get("digest"), str) and seal["digest"] == state_digest(snapshot), returning INTEGRITY_OK even when seal.get("alg") != "SHA-256".

Proposed Fix

• Reject duplicate JSON keys and NaN/Infinity constants in load_state (returning None).
• Pass allow_nan=False to json.dumps in sha_mapping.
• Check seal.get("alg") == "SHA-256" in check_seal, returning INTEGRITY_BROKEN when "alg" is present and not "SHA-256".