Zeroize transient seed material in Mnemonic::new

[mahzoun]

What

Mnemonic::new created two plaintext secrets and dropped them in place, leaving raw seed material in memory (notably WASM linear memory) until later overwritten:

• the raw 32-byte entropy buffer (crates/bitcoin/src/mnemonic.rs)
• the intermediate mnemonic-phrase String produced by generated_key.to_string()

This change wipes the entropy buffer with zeroize() once it has been consumed to derive the mnemonic, and holds the intermediate phrase in a Zeroizing<String> so it is cleared as soon as it is parsed.

Scope

Defense-in-depth, minimal change. It does not alter behaviour: the mnemonic stored on the struct and the values intentionally exported through the public API (as_string/as_words, WIF) are unchanged — those are by-design plaintext exports and out of scope here.

Notes

• zeroize (1.9) was already present transitively in Cargo.lock; this promotes it to a direct dependency of andromeda-bitcoin.
• cargo check -p andromeda-bitcoin is clean and the existing mnemonic unit tests pass.