Fix bridge startup with legacy artifacts

[appatalks]

Root cause

Eva 5.4 correctly requires a durable artifact epoch, but pre-identity releases stored files directly under the artifact root without an epoch. That verified legacy layout caused the bridge to fail closed with exit code 2.

Fix

• detect only the exact pre-epoch flat regular-file layout
• acquire the durable epoch lock across detection, atomic detachment, and initialization
• durably block the namespace before detachment
• quarantine legacy bytes without deleting them or re-authorizing them
• retain fail-closed behavior for missing epochs in current/nested stores

Validation

• complete nine-suite release matrix passed
• Action Plane: 174 tests passed
• real bridge startup integration against a temporary legacy store passed
• concurrent initialization test proves one migration and unique monotonic epochs
• migration failure test proves the namespace remains durably blocked