Publish governed Mutation Policy and Approval #29

Objective

Merge and publicly release the internally qualified .NET Mutation Policy / Approval implementation.

Internal candidate commit: 8d2ec666c42bd082676a5691249f70e6fb126ea8
Internal candidate version: 0.2.5-mutation20260930.0
Overall conformance tracking: https://github.com/teaql/teaql-conformance/issues/73

Acceptance

• whole-graph immutable Mutation Plan is reviewed before provider writes
• generated-default and missing-approval warnings remain backward-compatible
• matching policy identity approval is retained in audit evidence
• denial leaves zero provider writes
• focused tests and maintained example pass
• immutable public package is consumed from a clean cache
• public release evidence is linked back to the overall conformance issue