fix(passkeys): canonicalize backup state credential encoding

[pando85]

Summary

Fixes #111.

This PR makes APS use the same stable credential backup-state representation as soft-fido2 while preserving compatibility with credentials written by existing APS releases.

Canonical format

New writes emit exactly one CBOR text field:

backup_state: "notEligible" | "eligible" | "backedUp"

APS no longer emits backup_eligible.

Legacy migration

The decoder continues to accept the historical APS boolean pair:

backup_eligible: true
backup_state: false

Valid legacy combinations are converted through a typed three-state codec. The invalid BE=0, BS=1 combination, malformed types, unknown text values, and mixed canonical/legacy representations are rejected.

Credentials with no backup fields retain APS's established syncable-credential default (eligible). Migration is lazy: the next save rewrites a legacy credential using the canonical representation.

Compatibility

• The existing APS runtime boolean API remains source-compatible.
• Authenticator-data BE/BS flag behavior is unchanged.
• Full and metadata-only credential parsing use the same codec.
• The storage contract is documented explicitly under passkeys/CREDENTIAL_FORMAT.md.

Validation

The branch workflow runs:

./gradlew spotlessApply
./gradlew :passkeys:core:test

Regression coverage includes all canonical states, every valid legacy combination, invalid BE/BS, missing fields, malformed values, conflicting representations, metadata parity, and canonical lazy migration.

Related: pando85/soft-fido2#162