fix(crypto): harden secret key material against Debug leakage and zeroize gaps

[varex83agent]

Summary

Addresses five P0 secret-key-material findings across frost, crypto, and k1util — closing log-leakage holes, wiping in-memory secrets, and fixing one Charon parity divergence in on-disk key permissions.

High-value validator secret key material was held as plain byte arrays / bare stack scalars, printed via derived Debug, and (in one case) written to disk with world-readable permissions.

Changes

Task	Finding	Fix
T01	Derived Debug leaks FROST SigningShare/KeyPackage secret share	Dropped Debug from derives; added redacting manual impl fmt::Debug (<redacted> placeholder, public fields preserved on KeyPackage)
T02	Derived Debug leaks ShamirShare value	Manual Debug printing id, redacting value
T03	k1util::save() writes key world-readable	Unix path creates the file with 0o600 (and tightens an existing looser file), matching Charon app/k1util/k1util.go Save (v1.7.1, 0o600)
T04	Scalar not zeroized on drop; Copy conflict	Decision: keep Copy — removing it ripples across the crate for no real guarantee (the inner blst_fr is a Copy C struct, so moves bit-copy it regardless). Rationale documented on the type
T05	Ephemeral nonce & reconstructed key scalars never wiped	Explicit .zeroize() on k/wi/share_scalar in round1 and own_share_scalar/share_sum/share_scalar/total_scalar in round2
T06	Crypto crate holds BLS secrets un-zeroized	Added zeroize dep; wrapped raw ikm/candidate byte buffers in Zeroizing. Confirmed blst SecretKey is #[zeroize(drop)] (0.3.16), so BlstSecretKey intermediates already auto-wipe

Parity / safety

No wire-format, signature, or DKG-output changes — only Debug output, drop behaviour, and on-disk file mode. The only intentional parity change is T03 aligning the secp256k1 key file mode to Charon's 0o600. Zeroization is framed as best-effort defense-in-depth.

Tests

• 5 new unit tests: Debug redaction for SigningShare, KeyPackage, ShamirShare; 0o600 permission on save (incl. tightening an existing looser file).
• cargo +nightly fmt --all --check ✓
• cargo clippy on frost/crypto/k1util + dependent dkg crate, -D warnings ✓
• cargo test passes — including the 107 dkg tests, confirming the Debug-derive removals don't break dependents.

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