Security and privacy considerations - Context hygiene: keep…

Security and privacy considerations
- Context hygiene: keep contexts stable and collision-free (prefix namespaces, version fields). Example: brc-42:v1:payments:invoice:12345.
- Key separation: different context types should derive from distinct subdomains to reduce linkage risk.
- Address reuse: BKDS naturally encourages one-off receiving keys; enforce it in policy.
- Signaling: never leak raw xpubs; use context-based receive keys or signed requests.