Route back counter port

Definition

Hexagonal port for the per-issue route-back counter. Lives in src/route_back.py alongside RouteBackCoordinator. The coordinator depends on this port rather than StateTracker directly, so unit tests can wire a tiny in-memory dict implementation without pulling in the full state layer. Production wiring connects StateTracker as the concrete adapter.

Invariants

• Pure Protocol — no implementation, no state.
• Three methods: get_route_back_count reads the current count; increment_route_back_count returns the new count after incrementing; decrement_route_back_count rolls back an increment when a subsequent label swap fails, preventing transient network blips from burning route-back budget without any actual route-back occurring.
• decrement_route_back_count must be a no-op (returning 0) when the counter is already at zero.