Per-lane check capacity for dynamic scopes

The merge queue’s scopes.capacities option needs every scope spelled out by name, which does not work when your scopes come from a build tool and change with every pull request. The new scopes.default_capacity sets one budget that applies to any scope without an explicit capacities entry, so each lane runs its own speculative checks independently even when the scope set is dynamic.

merge_queue.max_parallel_checks stays the global ceiling on how many speculative checks run in total, and an explicit capacities entry still wins for its own scope.