Verify production laddr password-hash format before staging cutover

The laddr-import script writes legacy password hashes verbatim into `LegacyPasswordCredential` records, and the eventual account-claim endpoint will verify against them with whatever algorithm those hashes use.

Synthetic fixture data uses bcrypt (\`\$2y\$10\$...\`) per common Emergence-PHP conventions, but we haven't inspected real production hashes yet. Before staging cutover:

1. Pull the first ~20 \`Password\` values from a fresh production dump
2. Confirm they all share a single algorithm prefix (\`\$2y\$\`, \`\$2a\$\`, \`\$6\$\`, etc.)
3. If anything other than bcrypt appears, add a verifier in the account-claim plan and surface a warning in the import-laddr report

Filed as Follow-up from PR #24 (laddr-import).

laddr-import スクリプトから開始し、新しい production dump から最初のおよそ20個の Password 値を取得します。それらのアルゴリズムプレフィックスを比較します。単一の形式を確認するか、必要な verifier と警告を account-claim plan と laddr-import report に記録できれば完了です。

索引モデルが issue の本文から書いたものです。