refactor(endpoint): split EndpointInner god-object into named subsystems

[momics]

Summary

crates/iroh-http-core/src/endpoint.rs#L29-71 defines EndpointInner as a 17-field god struct holding pool, handles, serve_handle, stats atomics, event subscribers, path subscribers, compression options, etc. ADR-014 D1 named four named subsystems: Transport, HttpRuntime, SessionRuntime, FfiBridge. The split was deferred. Land it now.

Follow-up to the post-rework review at reviews/2026-04-30-post-rework-review.md §5.3.

Evidence

• EndpointInner has 17 fields covering five distinct concerns.
• Adding a new concern (a metrics export, a discovery hook) means another field on the same struct.
• Tests that need only the handle store have to construct or mock the full inner.

Impact

• High cognitive load — a reader touching one concern must scan 17 fields.
• Discourages independent unit testing of subsystems.
• Couples the lifetimes/visibility of unrelated state.

Remediation

1. Extract Transport (raw iroh Endpoint, node_addr, bound sockets, relay state).
2. Extract HttpRuntime (pool, max_header_size, max_response_body_bytes, request/connection counters, compression options).
3. Extract SessionRuntime (session registry, session events).
4. Extract FfiBridge (handle store, fetch tokens, drain/sweep).
5. IrohEndpoint becomes a thin façade composing the four; inner becomes Arc<EndpointInner> with one field per subsystem.
6. Public API of IrohEndpoint unchanged — the split is purely internal.

Acceptance criteria

• Four subsystem structs in their own modules (transport.rs, http_runtime.rs, session_runtime.rs, ffi_bridge.rs).
• endpoint.rs is the façade only — no business logic.
• Each subsystem is independently testable (an #[cfg(test)] new_for_test() helper per subsystem).
• All 92 interop pairs pass.