key-wallet: Seed is Copy + not ZeroizeOnDrop — the master seed is never scrubbed #843

Summary

key_wallet::Seed holds the 512-bit BIP32 master seed, but it is never scrubbed
from memory and can be silently duplicated:

// key-wallet/src/seed.rs
#[derive(Clone, Copy, PartialEq, Eq, Hash, Zeroize)]
pub struct Seed([u8; 64]);

• It derives Copy, so every Seed — and every implicit copy of one — leaves
  an un-scrubbed 64-byte master secret on the stack/heap.
• Because it is Copy, it cannot be ZeroizeOnDrop (Rust forbids Copy + Drop).
  So the derived Zeroize is never invoked automatically — the bytes linger until
  the memory happens to be overwritten.

This is the master secret from which every wallet key derives, so it's worth
holding to the usual secret-hygiene bar: zeroize-on-drop, and no silent copies.

Proposed fix

• Drop Copy from Seed; keep Clone (make duplication explicit).
• Add zeroize::ZeroizeOnDrop so the bytes are scrubbed when a Seed drops.
• Audit the sites that relied on implicit Copy and make copies explicit
  (.clone()) or pass &Seed — there are ~47 Seed references in key-wallet
  alone, plus downstream consumers.
• With Seed no longer Copy, from_seed_bytes (and friends) can take
  &[u8; 64], so a caller holding the seed in a Zeroizing buffer avoids an extra
  un-scrubbed by-value copy at the call boundary.

Blast radius / breaking change

Removing Copy from Seed is a breaking API change. Seed is used in ~47
places across key-wallet and by downstream consumers (dashpay/platform's
rs-platform-wallet, key-wallet-manager, key-wallet-ffi); every site relying
on implicit copy needs review. This warrants its own scoped PR (breaking title,
e.g. refactor(key-wallet)!:) plus a coordinated downstream rev bump.

Context

Surfaced from dashpay/platform#3841: create_wallet_from_seed_bytes wraps the seed
in Zeroizing but is forced to make a by-value copy to call from_seed_bytes. The
narrow attempt (#842) only removed that one call-boundary copy and did not
address the underlying Copy/no-scrub issue, so it was closed in favor of this
issue.

🤖 Filed with Claude Code