feat: tiered confidential-compute escalation contract (#22)

[mdheller]

What

Closes the §5.2 gap from the SourceOS-vs-Apple ModelCarry analysis: a tiered confidential-compute escalation. Model execution escalates to a higher confidential-compute tier (on_device → sealed_enclave → attested_tee) based on the data-residency / sensitivity class of the inputs, and the escalation decision is governed and receipted — the SourceOS analogue of Apple's on-device → Private Cloud Compute boundary.

Consume-not-fork; matches the existing manifest/boundary house style (schema + stdlib validator + valid/invalid fixtures + make target). Builds on ModelCarryManifest (#20, MERGED).

• contracts/confidential-compute-escalation.schema.json — ordered compute-tier lattice, sensitivity/dataResidencyClass → minimum-tier map (residency vocab aligned with the estate InferenceReceipt), and a receipted decision (chosen tier + reason + attestationRef + SHA-256 receipt).
• tools/validate_confidential_compute_escalations.py — structural validator, wired into make validate.
• docs/confidential-compute-escalation.md — contract, invariants, teeth.

SHA-256 is the authoritative receipt-hash algorithm (FIPS 180-4). Fail-closed is the core invariant.

Teeth

Fixture	Result
…attested-tee.json (valid)	VERIFIES — restricted-class request run at the attested TEE tier, at/above its required minimum, with an attestation ref.
…below-minimum-tier.invalid.json	REJECTED — restricted class (min: attested TEE rank 2) run on-device (rank 0). Fail-closed.
…missing-attestation.invalid.json	REJECTED — chosen tier requires attestation, decision has no attestationRef.
…non-monotonic-tiers.invalid.json	REJECTED — tier lattice not strictly monotonic by rank.
…unknown-class.invalid.json	REJECTED — request class not declared in sensitivityClassMap (never defaulted to weakest).

Verified locally: make validate and make release-dry-run both green.

Boundary / follow-up (@mdheller)

Structural contract only — no live TEE attestation, no compute provisioning, no runtime authorization. Follow-ups:

• Live TEE attestation verification (real enclave measurement / remote-attestation quote validation).
• Governed staged pre-load + atomic swap under a resource governor (Governed staged pre-load + atomic swap under a resource governor #21).

Refs #22, #20.