[New ADRs] Write ADR 0009: schema versioning and migration strategy

[ink-mami]

Context

The Data Model and Cryptography categories both need one authoritative decision record for how the emergency data model and attestation record version and migrate over time.

Task

Write an ADR formalizing the schema-versioning scheme and its interaction with attestation-hash verifiability across versions.

Acceptance Criteria

• ADR follows the standard format
• Decision consistent with the Data Model category's versioning-scheme issue
• Explicit interaction with attestation verifiability documented in Consequences
• Numbered 0009 and added to the index in docs/adr/README.md

Relevant docs

• docs/adr/README.md
• docs/data-model.md