Create raft/raft_fwd.hh with lightweight type aliases (server_id, group_id,
term_t, index_t) backed only by raft/internal.hh, avoiding the heavy
raft/raft.hh (832 lines with futures, abort_source, bytes_ostream).
Replace raft/raft.hh with raft/raft_fwd.hh in headers that only need the
basic ID types: tablets.hh, topology_state_machine.hh,
topology_coordinator.hh, storage_service.hh, group0_fwd.hh,
view_building_coordinator.hh, view_building_worker.hh.
Also remove gossiper.hh and tablet_allocator.hh from storage_service.hh
(forward declarations suffice), and remove unused reactor.hh from
tablets.hh. Add explicit includes in .cc files that lost transitive
availability.
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