[freshtonic]

Layer 2 of the CIP-5203 stack. Implements CIP-5304.

• KeyLifetime is returned with the index key, and the stack-encrypt keyset cache obeys it.
• Error::Backend { vendor, kind, source } and Error::ZeroKmsOnly are added, with refuse_zerokms_extensions for key vendor backends.
• IndexKey::from_key_bytes is a public constructor.
• stack-kms takes a breaking change (0.2).

Decisions made without review are in docs/plans/2026-11-05-key-vendor-backends-decisions.md. Read the CIP-5304 and orchestrator sections.

[coderabbitai]

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[github-actions]

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

caught	missed	unviable	timeout
20	0	12	0

Every mutant in the changed lines was caught by a test.

[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: stack-kms keeps only what a vendor needs: Error::Backend with a typed KeyVendor, Error::ZeroKmsOnly and refuse_zerokms_extensions, IndexKey::from_key_bytes, the stable vendor and mode bytes, and the ZeroKmsKeySource marker. Removed: KeyLifetime, LoadedIndexKey, SelectedKeyset, the per-settle index key, key_backend/keyset_backend/key_mode and RetrieveKeyPayload::key_mode. The key source traits are as on next. The stack-encrypt key lifetime work that was here is gone.

[freshtonic]

Replaced by #1632, 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.