Add Linux runtime crate and runnable single-user example

[sij411]

Description

Create the Linux runtime process that hosts a single local Feder actor and provides the foundation for later HTTP, storage, and delivery work.

Scope

• Add a feder-runtime-linux crate or equivalent workspace package.
• Load basic local actor and server configuration.
• Construct FederCore from runtime configuration.
• Start a minimal Linux process/server with structured logging.
• Provide a documented cargo run -p ... development command.
• Keep runtime-specific dependencies out of feder-core and feder-vocab.

Done when

• A developer can start the Linux runtime locally with one configured actor.
• Startup validates required configuration and reports actionable errors.
• The portable crates remain no_std + alloc compatible.
• Basic runtime startup tests exist.

Out of scope

• ActivityPub routes and WebFinger.
• Persistent storage.
• HTTP Signatures and federation delivery.