security: strengthen encrypted-backup passphrases and KDF policy

[emi-ran]

Scope

Improve resistance of portable encrypted backups to offline password guessing.

Required behavior

• Reject common low-entropy six-character passwords
• Adopt a stronger passphrase policy
• Raise KDF work factor based on measured supported-device cost, using a versioned format if required
• Retain bounded backward-compatible import for existing backups

Evidence

• BackupManager.kt:53-79,114-146

Tests

Weak common values fail; strong synthetic passphrase succeeds; legacy backups remain importable; excessive KDF parameters fail before expensive work.

Audit: 0a39f7e.