API reference: stealth key derivation deep-dive

[truthixify]

Extra labels: writing, api-reference
Tier: M (2-4 days) | Type: writing

Context. Same story for deriveStealthKeysFromWallet and its cousins. Users mix up scan-key vs spend-key + get subtle bugs.

Scope.

• Expand api-reference/stealth-keys.mdx.
• Signing message.
• Scan-key vs spend-key: what each does, when to use.
• Deterministic properties (why same seed → same keys).
• Passkey signer variant.

Acceptance.

• Snippets pass CI
• Scan-vs-spend confusion addressed with an explicit diagram
• Cross-linked with passkey guide

Files. api-reference/stealth-keys.mdx.