runtime/server.rs exceeds the audit file-length threshold (1534 > 800 LOC)

[nficano]

Category: refactor Severity: major
Location: crates/arcp-runtime/src/runtime/server.rs:1-3

What

At 1534 lines, server.rs owns builder configuration, handshake, transport loops, job dispatch, cancellation, subscriptions, artifacts, and utility helpers. This size makes spec-critical behavior hard to audit and has already hidden several missing conformance paths.

Evidence

//! ARCP runtime — the server side that drives the ARCP v1.1 §6.2
//! hello/welcome handshake (serialized as `session.open` / `session.accepted`)
//! and dispatches subsequent envelopes.

Proposed fix

Split server.rs along existing cohesion boundaries: handshake/session lifecycle, job invocation/cancel, subscription handling, artifact dispatch, and builder/configuration.

Acceptance criteria

• No extracted runtime module exceeds 800 lines, and the public runtime API plus existing tests continue to pass.