feat(attribution): pseudonymous file attribution (ESA-style)

Adapt Princess_Pi's Encrypt-Share-Attribution scheme so shared files are
anonymous but provably attributable.

A. In-session: a persistent Ed25519 persona key (~/.config/hack-house/
   persona_ed25519) signs every /send and /sendroom offer over the content
   hash; receivers verify it and see the persona fingerprint. Optional
   --attest <passphrase> attaches a revealable SHA-512(pass||sha256)
   commitment. Additive JSON — wire-compatible with the Python client.

B. Portable: /export-signed <dir> packages a directory into Princess_Pi's
   exact ESA 7z (fresh per-round Ed25519 sig over an inner 7z, SHA-512
   checksums, bundled verify scripts). Builder embedded from
   hh/tools/esa/esa_build.sh; verifiable with just bash+7z+ssh-keygen.

Tests: 45 pass (3 new persona). ESA archive build + verify-everything.sh +
passphrase reveal verified end-to-end.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

hh/Cargo.toml

@@ -19,6 +19,8 @@ fernet = "0.2"
 base64 = "0.22"
 rand = "0.8"
 hex = "0.4"
+# pseudonymous attribution (persona signing keys, ESA-style)
+ed25519-dalek = "2"
 
 # net
 reqwest = { version = "0.12", default-features = false, features = ["blocking", "json", "rustls-tls"] }