Wait for data mount before sessiond init

[posix4e]

Summary

• make dd-sessiond wait for /var/lib/easyenclave/data when its storage paths live under that mount
• prevents built-in recipe scripts from being written before mount-data hides the directory

Validation

• cargo fmt --check
• cargo check
• cargo test

Context

Dogfood authenticated POST /api/sessions returned Cloudflare 502 because the origin returned an upstream error: the advertised plain-shell recipe script did not exist after the persistent disk mounted.

[github-actions]

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