fix(key-wallet): zeroize ExtendedBLSPrivKey and ExtendedEd25519PrivKey on drop

[QuantumExplorer]

Fixes #858

Problem

ExtendedBLSPrivKey (key-wallet/src/derivation_bls_bip32.rs) and ExtendedEd25519PrivKey (key-wallet/src/derivation_slip10.rs) hold raw secret key material but had no Zeroize/Drop implementations, so their secret bytes were left behind in freed heap memory when dropped. Their secp256k1 siblings (ExtendedPrivKey, RootExtendedPrivKey) already zeroize on drop, so intermediate BLS operator / Ed25519 platform-node keys created during path derivation were the odd ones out.

Changes

• ExtendedBLSPrivKey: hand-written Zeroize (wipes the inner scalar via its public field — blstrs_plus::Scalar implements DefaultIsZeroes, and blsful's SecretKey itself has no Zeroize impl — plus the chain code and derivation metadata) and a Drop impl calling it, mirroring ExtendedPrivKey in bip32.rs.
• ExtendedEd25519PrivKey: same pattern — Zeroize wiping the [u8; 32] secret, chain code, and metadata, plus Drop.
• Fingerprint: added a Zeroize impl in bip32.rs (its inner field is private outside that module) so the new impls can wipe derivation metadata the same way the secp256k1 types do.
• Constant-time Eq for ExtendedEd25519PrivKey (secondary issue from the report): the derived PartialEq short-circuited byte-by-byte over the raw secret; replaced with a constant-time fold over the secret bytes (metadata still compared normally).

No caller changes needed: all existing field accesses are borrows or Copy-field reads, so adding Drop breaks no moves.

Testing

• New tests: test_zeroize_clears_key_material in both modules, and test_partial_eq_matches_field_equality for the hand-written Eq.
• cargo test -p key-wallet --all-features: 524 passed; the only failures are pre-existing wall-clock performance tests that also fail (worse) on unmodified dev on this machine.
• cargo clippy -p key-wallet --all-features --all-targets and cargo fmt --check: clean.
• cargo check -p key-wallet-ffi -p dash-spv --all-features: clean.

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📥 Commits

Reviewing files that changed from the base of the PR and between 1860089 and 7826a14.

📒 Files selected for processing (3)

• key-wallet/src/bip32.rs
• key-wallet/src/derivation_bls_bip32.rs
• key-wallet/src/derivation_slip10.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/zeroize-bls-ed25519-xpriv

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.66%. Comparing base (1860089) to head (7826a14).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #859      +/-   ##
==========================================
+ Coverage   73.62%   73.66%   +0.03%     
==========================================
  Files         324      324              
  Lines       73195    73268      +73     
==========================================
+ Hits        53888    53970      +82     
+ Misses      19307    19298       -9     

Flag	Coverage Δ
core	77.08% <ø> (ø)
ffi	46.07% <ø> (-0.02%)	⬇️
rpc	20.00% <ø> (ø)
spv	91.13% <ø> (+0.05%)	⬆️
wallet	73.15% <100.00%> (+0.09%)	⬆️

Files with missing lines	Coverage Δ
key-wallet/src/bip32.rs	80.74% <100.00%> (+0.03%)	⬆️
key-wallet/src/derivation_bls_bip32.rs	92.87% <100.00%> (+0.24%)	⬆️
key-wallet/src/derivation_slip10.rs	51.06% <100.00%> (+6.00%)	⬆️

... and 5 files with indirect coverage changes