Service manager step 4: Service scheme (kernel-side service: namespace)

Summary

Implement a kernel-side service: scheme with a flat namespace (service:/{name}) that brokers channel connections between service providers and clients. Protocol UUID handshake provides the real type safety.

This is step 4 of the service manager implementation (#26). Depends on step 3 (#29) for the protocol framework.

Desired behaviour

• Services register themselves at service:/{name} (e.g., service:/terminal, service:/keyboard)
• Clients open service:/{name} to get a channel endpoint connected to the service
• The kernel brokers the connection: creates a channel pair, gives one end to the client and one to the service
• The service and client then perform the protocol handshake over the channel
• Per-process registration: a process can register as a service provider, generalizing to future userspace drivers

Key files

• panda-kernel/src/vfs/ — new service scheme implementation
• panda-kernel/src/resources/ — service registry data structures
• panda-abi/src/lib.rs — any new opcodes for service registration
• libpanda/src/ — register_service() and connect_service() helpers

Design notes

• The service: scheme is intentionally simple — flat namespace, no hierarchy
• Type safety comes from the protocol handshake, not the scheme itself
• Registration is exclusive: only one process can register a given name at a time
• When a service process exits, its registration is automatically removed

Testing

• Kernel test: register a service, open it from another process, verify channel connection
• Kernel test: try to register a duplicate name, verify rejection
• Kernel test: service process exits, verify registration is cleaned up
• Userspace test: full round-trip — register, connect, handshake, exchange messages

Documentation

• Document the service: scheme in VFS documentation
• Add examples of service registration and client connection
• Document the relationship between scheme names and protocol UUIDs

Part of #26

Start by reviewing step 3 (#29) and the protocol framework, then inspect panda-kernel/src/vfs/, panda-kernel/src/resources/, panda-abi/src/lib.rs, and libpanda/src/. Implement the service registry, scheme operations, registration cleanup, and helpers described in the issue. Done means the kernel registration, duplicate rejection, exit cleanup, and userspace round-trip tests pass, with VFS documentation and examples added.