[freshtonic]

Layer 6 of the CIP-5203 stack. Implements CIP-5308.

• New crate stack-kms-gcp: a Cloud KMS REST client (encrypt/decrypt) that supports both key modes. Data keys are generated locally, the AAD goes in additionalAuthenticatedData, and CRC32C is checked on every field.
• Credentials: a static token, a service account JWT (RS256 with the rsa crate), and the metadata server (native only).
• Tests: hand-written fixtures, a JWT known-answer test against openssl, and the contract suite.
• The opt-in live workflow is integration-gcp-kms.yml. To turn it on, set the variable STACK_KMS_GCP_LIVE_KEY and the secret STACK_KMS_GCP_SERVICE_ACCOUNT_KEY.
• Before release: a security reviewer must sign off on RUSTSEC-2023-0071 (rsa). See the orchestrator review in the decision log.

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[github-actions]

Mutation testing (cargo-mutants, --in-diff, stack-auth + stack-encrypt)

No mutants were generated for the changed lines.

[freshtonic]

Scope reduced (2026-11-12). This branch was rebuilt from one final tree by path, and the follow-up PRs #1555 to #1563 were folded in. The plan is docs/plans/2026-11-12-key-vendor-backends-scope-reduction.md; the decision log's "Scope reduction" section records each change. Decisions: no stack-encrypt change, no v2 leaf, and the wrapped data key mode deferred.

In this PR: no same_key. The AAD fixtures were computed again by a separate script. This PR now also brings _test-kms-live.yml (#1556).

[freshtonic]

Replaced by #1628, in the new stack #1633. The code is the same, rebased onto the latest next. The ADRs are now ADR-0013 and ADR-0014, and the text about the wrapped data key mode is removed.