Detect pending legacy profile convergence

[Howe829]

What changed

• Load the local DID document before reporting VNext legacy-upgrade completion.
• Return completed only when foundation profile discovery no longer requires convergence.
• Allow identities with legacy draft profiles to enter the existing root-signed convergence workflow.

Root cause

legacy_upgrade_status treated every VNext device state as completed. Identities that still carried legacy draft foundation profiles therefore skipped the existing convergence path and later failed device approval because those profiles are read-only.

Impact

VNext identities with canonical profiles remain a fast completed path. Legacy draft profiles now resume the existing idempotent convergence flow while preserving authority and device identity.

Validation

• cargo fmt --all -- --check
• cargo test -p awiki-im-core identity_legacy_upgrade --locked — 12 focused tests passed
• Android arm64 native SDK rebuilt successfully