-- Only an acknowledged provider destroy may create retirement evidence.
-- A changed PID namespace, provider name or generation is not sufficient.
CREATE TABLE computer_generation_retirements (
    computer_id TEXT NOT NULL REFERENCES computers(id) ON DELETE CASCADE,
    generation INTEGER NOT NULL CHECK(generation > 0),
    provider_ref TEXT NOT NULL,
    home_key TEXT NOT NULL,
    proof_id TEXT NOT NULL UNIQUE,
    proof_kind TEXT NOT NULL CHECK(proof_kind = 'provider_destroy_ack'),
    acknowledged_at TIMESTAMPTZ NOT NULL DEFAULT clock_timestamp(),
    PRIMARY KEY(computer_id,generation)
);

-- Historical artifact rows did not record their verification source; do not
-- retroactively label them as execution receipts without trusted evidence.
ALTER TABLE computer_artifacts ADD COLUMN verification_basis TEXT NOT NULL
    DEFAULT 'legacy_unspecified'
    CHECK(verification_basis IN ('legacy_unspecified','execution_receipt','postcondition'));