Add secure erase for sensitive memory in WASM scanner

[collinsadi]

Derived stealth keys may linger in WASM linear memory after use.

Zero sensitive buffers after derivation in scanner Rust code and document limits of browser memory.

Acceptance criteria:

• Sensitive buffers zeroed on drop.
• Document states browser memory limits.
• No regression in scan performance.

[Vicsygold]

@Vicsygold has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Pls i beg you can i have this project i will complete ASAP. God bless you richly!

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Vicsygold to this issue.

[ndyugwu]

@ndyugwu has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

i would like to work on this kindly assign me

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ndyugwu to this issue.