docs(architecture): spec alknet-core with per-crate subdocs, ADR-010/011
Add alknet-core architecture specs in docs/architecture/crates/core/ with
focused subdocuments for core types, endpoint, auth, and config. Write
ADR-010 (ALPN Router and Endpoint) defining AlknetEndpoint, HandlerRegistry,
accept loop, and graceful shutdown. Write ADR-011 (AuthContext Structure)
defining AuthContext fields, immutability in handle(), and IdentityProvider
injection pattern. Resolve OQ-04 (static registration), OQ-12 (file paths
only for v1). Add OQ-11 (auth observability). Fix remaining alknet-secret
references to alknet-vault across ADRs 003/004/005/009.