internal/escrow: zero-knowledge escrow creation. R = 10 EFF-wordlist words
(crypto/rand, ~129 bits); wrap K under R via PBS-native key change-passphrase
driven over a stdlib pty (x/sys/unix; output discarded so R can't leak, F-A2);
self-verify the blob recovers K (fingerprint match) before shipping. Opt-in (b)
R-wrapped offline copy + (a) raw paperkey. Live K is byte-unchanged (operates on
a copy). --selftest=escrow-create (-storage/-paperkey/-offline/-upload). Posture
config field (zero_knowledge default). PBSEncKeyPath helper. Grounded by the
escrow spike findings.
Tests: R entropy>=128/format/uniqueness; integration round-trip (wrap->unwrap
fingerprint match, wrong-R fails, K byte-unchanged) guarded to linux+pbc.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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